Personally I think, HELL YEAH there is a need to jailbreak after 3.0. And here's why:
1) Unlock iPhone to use in other carriers. If u ever travel overseas and want to use a local SIM, u can't unless u unlock. Ever see at&t International calling rates??
3) Recently at&t and Apple have been in cahoots on limiting 3G functionality. Apple has been forcing developers to limit some apps to ONLY wifi. ie, Skype, Slingplayer. I am sure there will be others in the future (HULU?). When u jailbreak, u can fool the apps to THINK they are using wifi when they are really using 3G.
4) better interface. I like Categories where I can put my apps into folders by type. A NEWS folder for my news apps. A GAMES folder for all my games.
5) Tethering - Will it happen under 3.0? Will it be free like it is under jailbroke iPhones?
Just a few reasons to still jailbreak.