We've extensively covered an issue in which in which some third-party applications crash or do not launch for many iPhone OS 2.0.x users. Previous reports indicated that the issue can be eliminated by reducing the number of applications installed on the iPhone. We also noted a separate fix that involves downgrading to an earlier version of iTunes and another more involved fix.
Further investigation reveals that the issue may be solved by simply deleting all applications from the iPhone (via a restore process, if desired), then re-downloading all applications through the iTunes store on a desktop computer instead of though the AppStore on the iPhone itself.
If you've had success with this fix, please let us know.