I'm not able to do this because I do not have Outlook, but I've read there are ways to merge events (birthdays, holidays, etc from subscriptions cals) into your main using Outlook. It'll then resync back to WLive. You can also set your main calender to, say, your birthday calendar, and have that as the one that syncs to the phone. You can do this in settings on mail.live.com.
I'm interested in other alternatives and have been looking since I got my phone. If anyone knows any other methods, please share! Also, necrobump.