Update: We have a fresh list of Cydia apps working / not working on iPhone 3.0 here.
Getting Cydia on your iPhone 3.0 is now a breeze. Though the OS is still in beta, a special version of QuickPwn with Cydia 3.0 is out. This will jailbreak OS 3.0 for you and bring the 3rd part apps along with all the Apple official features. The problem is that v3.0 is beta, not official, and for iPhone 3G users, it will mean losing the software unlock (yellowsnow).
You can download the new quickPwn from here, here, here(rapid share) or here
Here, is a list of Cydia softwares that work and do not work on iPhone 3.0 beta.