You WILL Be able to upgrade without any issues to the new firmware.
Ironically enough iPlus/iLiberty users may have issues however!
Let me explain!
3.9 came on all original US models - so the 2.0 firmware will work as it does now.
Obviously if your not on an official iPhone contract then you won't be able to activate the phone unless it is jailbreaked.
It seems very likely that within hours of it coming out a jailbreak method will be released as the dev teams have already got into the beta.
However if you used iPlus/iLiberty to UNLOCK (and your phone was 1.1.2 or newer out the box)- then you will have the 3.9FB bootloader. IF Apple includes other bootloader files with the firmware update then there is a chance that it will mess up the phone in someway as is happening with restoring to 1.0.0 and 1.1.1 firmwares now (ie going from 3.9 to 3.8).
Having said that the beauty of this is that if you have used iPlus/iLiberty that you can go back to 4.6 before you update although you would lose the unlock again so that would only work if you have an official iPhone contract again!
Of course if there is no older bootloader files in 2.0 then there is no reason why you can't upgrade to 2.0 - although again unless your on an official contract yourl be locked out!
However this whole argument seems pretty redundant as a news article on iPhone atlas from yesterday seems to suggest you can upgrade and keep your existing unlock if you use the devteam method!
http://www.iphoneatlas.com/2008/03/26/iphone-os-20-may-leak-publicly-soon/
I think the point here is that its best to wait for a dev team release before upgrading simply because even if you have an official iPhone contract you may as well jailbreak and unlock if possible at the same time as upgrading! as it seems that the DevTeams are hot on it!