The most obvious time to check, of course, is that the "Sync iCal Calendars" has its checkbox 'checked.' Then, if the "Selected calendars" radio button is selected, there should be at least one actual calendar check marked. Since I don't have an iPad, I don't know if there are any settings on it that could affect this (lack) of behavior. On my iPhone, I don't know of anything that would prevent syncing, as long as the device shows up in iTunes, of course.

Hope this helps!