cannot sync, not enough free space. 'other'is taking up too much space. i have no 'other' things on my phone and have never had this problem before. I believe this may have come from upgrading to ios5.
How big is "Other"? On iOS 5 it is typically about 1 GB. On earlier versions it was 300-500 MB. If yours is much larger than 1 GB you have corruption in the file system on your iPhone. You can fix it by restoring the phone with iTunes.
Connect to iTunes, then click the Restore button. When asked, choose to set up as a new phone. After it finishes, right click on the name of the iPhone in the list on the left side of the iTunes window and choose Restore from Backup. Do this BEFORE you sync and after restoring.