What Happens when MobileMe Expires?

AFAIK, the Apple ID will still work even if it's an expired MobileMe address.

You might need to change your email address on that account though.

Any receipts from purchases, lost password retrieval etc won't work ... it would be risky to leave it tied to a dead MM account .. you can easily change the address it points to .. i would do so before it expires to be safe

Your computer will enter a self destruct sequence that can only be deactivated by Mr. Jobs himself... and another paid subscription to MM.

On to real business, I would change it just to be safe. In my thinking, if your not paying for it, Apple won't let you use it.