Ok so the issue is user specific. So nothing wrong with iCal here. One or more files in your user account have been modified and their are causing iCal's icon to behave strangely.
Go to Apple Menu > System Preferences > Language & Text > Formats > Region. Make sure this has a country name in it, not "custom".
If that does not resolve the issue then:
Backup iCal by going to File > Export > iCal archive.
Quit iCal.
Remove the following to the trash and restart your computer:
Home > Library > Caches > iCal
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
__NOTE: Removing these files will remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.