If you have an iPhone 3G and haven't restored it yet, do so. The first batch of iPhone 3G units shipped with iPhone OS 2.0 5A345, but the current build is 2.0 5A347. A handful of bugs were squashed with the new iteration, which brings enhanced stability and other enhancements.
Unfortunately, Apple hasn't designed iTunes to recognize the difference between the iPhone OS 2.0 builds. As such, you'll need to perform a "Restore" function to pull the new release.
In order to restore, connect your iPhone to your system then click the Restore button under the Summary tab in iTunes. Restoring the phone will erase contacts, calendars, photos and other data on the phone (including any third-party applications), but will restore automatically backed-up information including text messages, notes, call history, contact favorites, sound settings, widget settings, etc.
The restoration process itself may eliminate some iPhone issues.