Jailbreaking really is the way to go. You really don't wanna lose that grandfathered unlimited plan unless you know for a fact you'll never go over 2gb of data. It's really not a difficult thing to do. As mentioned above, check out iClarified.com or a handful or other sites to properly set the expectation and just be smart about reading up on it. It doesn't seem like you're the type of person to rush so I'd say you'll be fine. Just always back up...That's what I thought. To be honest, I use less than 1GB of data per month, so I don't really need the unlimited plan. But I hate the thought of giving it up, just in case!
Then again, I'm not too crazy about the thought of jailbreaking my phone and keeping up with potential updates... so the tethering/hotspot option might be best for me. Who knows?
So, it's either $30/month to keep the unlimited plan and jailbreak the phone...
Or $45/month to use the tethering/hotspot option with 4GB data max.
OR, get the 3G iPad and only buy the data plan when I need it, by the month.
You can start here:
http://iclarified.com/entries/index.php?caid=2&scid=11&seid=2