You have nothing to worry about. I've been using my expired .Mac address for 2.5 years now, without any issues.
Apple lets you keep your .Mac address as your AppleID even after .Mac expires. That way, if you ever decide to get .Mac again, you can use that old address, without having to create a new one. Once you register .Mac, and choose an ID, that ID becomes yours for the rest of your life.