Some users have reported an issue in which iTunes 10 for Mac OS X crashes on launch. There are a few potential causes for this issue, and corresponding fixes.
Remove third-party add-ons. Start iTunes in safe mode by holding down the Option and Command keys (Mac) or the Shift and Control keys (Windows) while launching iTunes. If this solves the problem, try removing third-party plug-ins as described in this Apple Knowledge Base article.
Also, remove any third-party items from the folders /Library/QuickTime and ~/Library/QuickTime.
In particular, some users report that removing DesktopVideoOut.component from these locations resolves the issue.
Remove .plist files. Quit iTunes, then move the following files to a different location (the ~ represents your user folder):
Relaunch iTunes and check for proper startup.
Recreate your iTunes library. As described here.
Remove then reinstall iTunes. Failing the above, try completely removing iTunes per these instructions then reinstall iTunes 10.
Downgrade to iTunes 9.2.1. If both of the above options fail, you can (as a last resort) downgrade to iTunes 9.2.1.