Service Pack 1 broke my color coded calendar code. (http://techtrainingnotes.blogspot.com/2010/06/sharepoint-2010-color-coded-calendars.html). The fix is a simple edit, just change SP.UI.ApplicationPages.CalendarNotify.$4a to SP.UI.ApplicationPages.CalendarNotify.$4b.
I am still going through my SP 2010 customizations, but so far, the color coded calendar is only one that SP1 broke.
Do you know of other changes?
If you have found other SP 1 changes that might impact client side customizations, please post them as a comment below!
Do you have SP 1?
If you get a popup showing “undefined”, then you have SP 1 (or later). If you see code, then you have a pre-SP1 installation.