Found this in the SDK read me
• A small set of AT&T customers may experience data services that stop working
after installing iPhone OS 3.0 beta 5 on a device. If you experience this problem
with a device that worked properly before the beta 5 update, please call the
developer support line at 1-800-633-2152 for help restoring data services.
I've loaded it, and am syncing now. For the last two betas I have read people are updating (rather than restoreing to the new betas) and no issues. I tried it this time with the new prerelease iTunes and all my music and most of my apps got pushed into the "other" category. Hopefully a sync will work that out.
*edit* Sync didn't work. Will most likely have to restore.