Why isn't iPhone syncing Calendar items with Outlook?
The best troubleshooting I have found for this problem is iPhone, iPad, iPod touch: Troubleshooting contact and calendar syncing via USB on Windows(Apple), mainly due to the fact that it explains how to reset sync history in iTunes, a process that reevaluates the contents of both devices and tries to resync them.

Another good little trick is to try a "Replace data on this iPhone" near the bottom of the info tab, that will wipe the specific data set off the iPhone and replace it with the computers, often a good way to get them talking to each other again.