"Mac OS X 10.5: Subscribed calendars are listed in their own section of the iCal calendar list. When syncing subscribed calendars to MobileMe, the calendar information and settings for the calendar is synced, but all of the downloaded events are not. This is done to help streamline the syncing process with MobileMe, as it prevents syncing redundant information to other computers with an Internet connection."
In the past... I have had to re-subscribe to calendars for them to start working again.
For birthday calendars (originating from your contacts in address book) if you un-check show birthday calendar in iCal preferences, close iCal, launch iCal, then re-check sync birthday calendar... This worked for me.
I got my birthdays to show up on my online mobileme calendar.
As far as I can tell, there is no other way besides exporting/importing described in #8 above to add a "subscribed" calendar
Here's what I did...
I opened my online MM calendars
Clicked the little gear box at the top of the screen
choose prefs and click show birthdays
log out and back in and birthdays show.
NOW, the source of this birthday calendar is interesting. I think it's pulling from online contacts. That means that if you update a contact birthday and sync the contact up, the birthday calendar gets update. Or, if you update the contact online it happens right away.