Okay, there's a lot of threads and information (true and false) floating around out there, so I think we need to get everything straight and in one place because the number of threads is getting a little out of hand.
This is not a place to find out where to download the beta. If you want it that bad you'll be able to find it on your own.
Download the beta
So first thing you need to do is download the beta from your Dev account.
Signing up for an Account
If you're only signing up to try 3.0 I would reconsider. But it's your money, so do as you please. Once you sign up Apple will send you an e-mail authorization code. This can take up to a few hours so be patient.
Find your UDID number
Before installing the 3.0 beta YOU MUST register this number with Apple in your Dev account. The UDID can be found in iTunes. With your phone plugged in click on "Serial Number" and it will change to the UDID number. Also you can go to System Profiler, click on USB and click on iPhone and it will be listed as "Serial Number." I found this way easier because you can copy/paste. Again, do this before you install 3.0
Installing 3.0
So there may be a different way to do this, but what I did was option-click on Restore and selected the 3.0 .ipsw and was good to go. If you DID NOT register your UDID number then when the install is complete you will not be able to sync with iTunes. This is why it is important to do it before. It is possible to just enter the UDID after and it will begin to function normally but why create issues for yourself - just do it first.
For Non-Developers
I'm sure most of the people curious about 3.0 are non-Developers. Again, this isn't a place to discuss where to get it. But if you do figure out where and find yourself with a copy, do not just install it. You need that UDID registered as I posted above. Some developers are willing to add phones for free, some are charging and some people are creating a new Dev account and splitting the cost. If you really want this that bad I suggest you make some new friends.
This is a BETA
So it will work with like a Beta. Some things crash, some apps wont run, YouTube doesn't work if you restore form a previous backup. If you seriously need this phone to function I would not suggest upgrading. After all, this is for Developers to test their apps not for the general public to run.
Downgrading
I've heard it IS possible to downgrade, however I was not successful when I tried but many people have claimed successful so I probably just did something wrong. EDIT: Seems to work on PC's but not Macs. I will try it tonight in Parallels/Boot Camp and see if I am successful. But you can downgrade. The steps are
1. Attempt a restore of 2.2.1 in DFU Mode
2. Get the iTunes error message
3. Reset device
4. Launch QuickPwn
5. Press next and get past the first 3 steps
6. Phone is fixed
If you really are that worried about downgrading then you probably shouldn't be installing this in the first place.
I think that's all, if there's still questions or I mis-stated something please let me know. I want this to be as helpful as possible.
This is not a place to find out where to download the beta. If you want it that bad you'll be able to find it on your own.
Download the beta
So first thing you need to do is download the beta from your Dev account.
Signing up for an Account
If you're only signing up to try 3.0 I would reconsider. But it's your money, so do as you please. Once you sign up Apple will send you an e-mail authorization code. This can take up to a few hours so be patient.
Find your UDID number
Before installing the 3.0 beta YOU MUST register this number with Apple in your Dev account. The UDID can be found in iTunes. With your phone plugged in click on "Serial Number" and it will change to the UDID number. Also you can go to System Profiler, click on USB and click on iPhone and it will be listed as "Serial Number." I found this way easier because you can copy/paste. Again, do this before you install 3.0
Installing 3.0
So there may be a different way to do this, but what I did was option-click on Restore and selected the 3.0 .ipsw and was good to go. If you DID NOT register your UDID number then when the install is complete you will not be able to sync with iTunes. This is why it is important to do it before. It is possible to just enter the UDID after and it will begin to function normally but why create issues for yourself - just do it first.
For Non-Developers
I'm sure most of the people curious about 3.0 are non-Developers. Again, this isn't a place to discuss where to get it. But if you do figure out where and find yourself with a copy, do not just install it. You need that UDID registered as I posted above. Some developers are willing to add phones for free, some are charging and some people are creating a new Dev account and splitting the cost. If you really want this that bad I suggest you make some new friends.
This is a BETA
So it will work with like a Beta. Some things crash, some apps wont run, YouTube doesn't work if you restore form a previous backup. If you seriously need this phone to function I would not suggest upgrading. After all, this is for Developers to test their apps not for the general public to run.
Downgrading
I've heard it IS possible to downgrade, however I was not successful when I tried but many people have claimed successful so I probably just did something wrong. EDIT: Seems to work on PC's but not Macs. I will try it tonight in Parallels/Boot Camp and see if I am successful. But you can downgrade. The steps are
1. Attempt a restore of 2.2.1 in DFU Mode
2. Get the iTunes error message
3. Reset device
4. Launch QuickPwn
5. Press next and get past the first 3 steps
6. Phone is fixed
If you really are that worried about downgrading then you probably shouldn't be installing this in the first place.
I think that's all, if there's still questions or I mis-stated something please let me know. I want this to be as helpful as possible.
