Something to be aware of if you are supporting a large number of iPhone’s is the potential bug Apple has run into. It does not appear to effect the all-important calendar or e-mail functions, but for some people using repeating alarms this may an hour delayed after the end of daylight savings time (DST).
Please see the Apple knowledge base article here: http://support.apple.com/kb/TS3542
It appears the only workaround is to stop the repeating alarms until after November 7th, 2010. I would almost assume that this slight bug will be fixed in the upcoming iOS 4.2..