You can't change your current apple id to a me.com id, and you can't merge both ids.
However, there is a way to consolidate all your purchases under one id (assuming you can get them all on the same computer). There's a thread on apple support forums talking about it.
I think I did the process, not exactly sure though. However, I am able to now use my me.com id as my primary apple id and it seems like I can access all my purchased apps with my me.com id rather than the hotmail id I was using before.