Here is what I have come up with so far;
A good place to start is Apple.com support.
MacWorld.com also has a good troubleshooting article, titled Troubleshooting iPhone and iPod touch issues. From the article;
In increasing order of difficulty, here are the things I do to try to recover from the various iPhone 2.0.x maladies I’ve experienced:
- Restart the iPhone. Press and hold the power button until you see the “slide to power off” display, then do just that. Let the phone site for a couple seconds, then power it back on. I’ve found this is generally effective at solving excess battery consumption, as well as the slow iPhone problem. It only takes about a minute, at most, to reboot your iPhone, so it’s by far the easiest thing to try.
- Reinstall a third-party application. If one particular third-party program is giving you troubles, you can try reinstalling it. First delete it from the iPhone by tapping and holding on the program’s icon, then clicking the “x” button to delete it. In iTunes, re-download the program (if you’ve bought it once, you can do so for free), then sync your iPhone and install the new copy of the program. Warning: You will lose all data you saved with the program—text you added, high scores you set, progress through the game that was saved, etc.
- Restore the iPhone, including its backup. Unfortunately, this painful solution may wind up being your most-used troubleshooting technique. If you’re experiencing issues with multiple applications, or with Apple’s applications, or if neither of the above steps solve your problem, then a restore may be in your future. Before you restore (as explained above), however, sync your iPhone one more time and let it run a full backup (assuming your iPhone is in a usable state). This way, at least, you’ll be able to save any of the data you’ve stored with your third party programs. After the iPhone’s software is restored, iTunes will then offer you the chance to restore from the new backup.
- Restore the iPhone, start from scratch. If you try the restore method above and find you still have the same issues as before, then you’ll have to use this most painful troubleshooting method. Restore the phone as above, but do not restore your backup. It’s possible that one of the files in the backup is corrupted, and that’s what’s causing your issues. Instead of restoring the backup, tell iTunes to treat your iPhone as new. Give it a name, reinstall all of your programs from scratch, and then sync your music, videos, and other content. This is as close as you can come to starting with a factory-fresh iPhone, and represents your best chance at stability—at least for a little while.
TUAW.com calls their troubleshooting guide iPhone Troubleshooting: How to Restart, quit frozen apps and Reset iPhone.
MobileMe users can find troubleshooting guide lines in an article titled MobileMe: Troubleshooting iPhone or iPod touch sync issues at Apple.com.
To get some of the most up to date info Apple.com's iPhone discussions board is invaluable.
In closing I was not really trying to be comperhensive here, I just wanted to provide a few good places online to go if you are having problems with your iPhone.