Good news and bad news!
Good news is that 2.1 and all future updates to the iPhone are jailbreakable (and unlockable on the original iPhone) unless Apple changes the hardware.
They didn't on the iPhone 3G and so that is jailbreaked with the same method.
As the previous poster said 2.1 beta is already jailbreaked by the devteam.
Obviously you do have to wait for an update to pwnagetool to be released before you can update if you want to stay jailbreaked...although there is no reason why you can't update officially then just jailbreak afterwards.
Now the bad news!
If you decide to jailbreak 2.1 you will have to RESTORE in order to make sure everything is working perfectly.
There is nothing stopping you clicking on update and selecting the custom firmware 2.1 but the jailbreaked settings in previous jailbreaks and updates have caused issues as they conflicted - likewise you can't restore from backup as you will likely get random problems with the iPhone.
By that I mean it may lock up it may crash - jailbreaked apps may not work properly - etc. It can be fixed by a restore and setting up the phone as new.
It is not a big deal except you will lose all your data.. and have to resync music etc.
Let me be clear here - you can update and restore from backup - but the chances are it will NOT work properly.
It may be fine for some but if you decide to restore from backup or update direct to 2.1 custom remember that you will almost certainly get problems and have to do a fresh restore.
It has been like that with every jailbreak and firmware update!