That was a rather annoying problem.
And rather odd.
iCal has been crashing for a few weeks on my Mac Mini running 10.7.2 - a bit of Googling led me to think it was corrupt data. I use Google Apps for mail and calendaring, so I followed an article that suggested removing the accounts and deleting all the cache information in ~/Library/Caches.
That fixed the problem for a couple of days, then it was back again. Since it was fairly intermittent I just put up with it.
Then today I couldn't open iCal at all. Or Mail. Or the System Preferences panel for Mail, Contacts & Calendars. A lot more chasing my tail ensued whilst googling the crash information.
To cut a much longer story short, the fix turned out to be deleting MobileMeAccounts.plist from ~/Library/Preferences.
Surreal, quite surreal.