Changing my iTunes account to a mobile me email address

I think I had the same issue as you a while back. Essentially, you want to "merge" your purchases from your old AppleID (and iTunes Store account) to your new MobileMe account (which also serves as an AppleID). You're stuck in the position where you have two AppleID's and want to keep all your purchases in one place.

I searched around forums and such when I was having this issue and the solution is to call Apple support and tell them what you want to do. When I did it, they told me that I couldn't merge my purchases into MobileMe, but they showed me a way to edit my old account information so it would be less confusing. I used http://myinfo.apple.com to change the name of my old account to something other than my old email address (AppleID's don't have to be email addresses, they can just be a single username) and then changed the contact email associated with my old account to my new MobileMe email. This way, I still had two accounts, but my old email was no longer involved. After you do this, continue to use your old account (but with the new name you've made for it) to buy all of your music. Only now, all of your receipts and whatever else will be emailed to your MobileMe account.

If you choose to make this change like I did, just continue using your old AppleID (with your newly created name) for everything you do except logging in to the MobileMe website, and of course setting up syncing between your devices. This way, if you ever choose to discontinue MobileMe, you avoid any problems you might have with music you bought using it, because you've been using your "old" one the whole time (which you can just edit the contact info for anytime you wish).

That's basically what Apple told me on the phone. I know it's kind of confusing. Let me know if you need clarification. The Apple support people were actually very helpful when I called and asked them about this, so they could clarify as well.

On a side note, while this is the best available solution to my knowledge, I still think this is a really confusing issue for people signing up with MobileMe. Apple should have some kind of merging or simplification process.