Well, without any doubt about it, the 3.1 OS update utterly corrupted my iPhone on Tuesday. My handset's only 120 days young and barely even had, at the most, a dozen of the most popular, stable Apps installed.
3.1 instantly threw my iPhone into a permanent state of Recovery Mode and it will not restore. I have tried every step in the support article twice and iTunes returns different unknown errors each and every time (
http://support.apple.com/kb/TS1275).
I think it only takes 5 mins of looking on these and other forums at the sheer volume of people suffering corruptions and instability across all applications and features as a direct result of the 3.1 install.
In my call to AppleCare support today the tech operator told me in not so many words that every call she was taking was about faults presenting after updating to OS 3.1 - and I had to queue on the line for 40mins before I even got through today at 6pm GMT.
Conclusion: iPhone OS 3.1 is like a bad Windows ME (and that's being generous).
If you haven't updated, don't! Wait for the fix; it HAS to come in short order.