As you tested in a new user we know this is not an issue with iCal, rather with some piece of data in your user account. Good testing!
1. Backup your data: http://support.apple.com/kb/HT1553
2. Remove the following to the trash and restart your computer:
Home > Library > Caches > com.apple.ical
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache if present)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
—NOTE: Removing these files may remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.
—NOTE: To get to “Home > Library” in Lion hold down the option key and click on “Go” while in the Finder.
3. If the issue persists boot into Safe Mode and test again and report your results: http://support.apple.com/kb/HT1455
People hate the interface for the above two programs. I don’t blame them. I don’t use those applications, but if I did I would use kurosawa99 suggestion below.
Matthias Eckardt’s free, “LionBleacher” helps some. At least it eliminates the horrible “Leather” metaphor.
I was bit by this “feature” today. I accidentally sent a personal email to someone who I didn’t intend to sent it to. If you have a Mac laptop, sometimes the track-pad is activated just by being physically close to it. I don’t know if it is heat or what. So often you select things that you don’t want to select. I have ignore accidental track-pad input selected, but the track-pad still is sensitive. It almost makes you need to buy a magic track-pad. Perhaps in the future.
So how can you avoid this feature? You can download and install Zimbra desktop for Mac. I am using it now and it has some really great features. For example, if you connect your LinkedIn account to it, it will search LinkedIn for the addresses you have in your contacts there. Talk about a great feature. Not having to synchronize those lists are great. Having control to set preferences the way you want is great as well. It also works with other email systems and is a fantastic option for business/personal use. I am very impressed.
Christopher Breen comes to the rescue with this article Mac troubleshooting FAQ: iCal and Mail. These are some common problems that I have read people on the Apple Discussion forms have. Is everyone excited about the next version of Mail in Lion? Doesn’t it look amazing and useful?