It seem iOS 4 create many problem to all users of Iphone so don't do an update first.

All contacts deleted	1788 (4.6%)
Random crashes	1943 (5.0%)
Heavy slowdown	2960 (7.6%)
MMS messages aren't sending	232 (0.6%)
Google Sync issues / unable to retrieve Gmail	1662 (4.3%)
Music and videos won't restore after the upgrade	600 (1.5%)
Can't connect to data networks	957 (2.5%)
Photos aren't rendering properly	783 (2.0%)
Other (let us know in the comments!)	2607 (6.7%)
No problems so far: installed without a hitch and running just as fast (if not faster)	15131 (38.8%)
I just like to push buttons, wouldst thou call me a monster?	10368 (26.6%)

As of 2.25pm (+8GMT)


http://www.engadget.com/2010/06/21/poll-is-ios-4-causing-you-grief/