I too, have 2 apple IDs, my gmail address which I have used to make iTunes purchases for years and my @me address. For "all things apple" I have wanted to use my me.com address for a while. This is what I did:
1. Every time I made a purchase on iTunes I burned the music to a back up CD. So the protected music I would always delete the original purchase and copy it back to iTunes from the CD. This left all my music in iTunes unprotected. I could play it without logging in and move it around on my computers more easily.
2. Once Apple started offering the purchase of unprotected music, that is what I always purchased for the above reasons.
3. I then copied ALL my retail store purchased CDs into iTunes so my entire music collection is digital.
4. Since it is unprotected, I can sign in with my me.com address and from now on make purchases using that.
I also purchased iTunes Match with my new address so my entire music collection is in the cloud. It worked great.
That doesn't work for movies, of course, but I only had a few that I purchased through iTunes, I would usually rent movies, so the impact on that was minimal. I've used Amazon for instant video purchases for quite a while, more flexible than iTunes.
The App store is a little different, I have to install old apps logged in with the old gmail ID. I then signed out and signed back in with my me.com account and make all new purchases with that. Because of the beauty of Time Machine I have rarely had to log back into the Mac App store to reinstall something. I just copy it from my time machine backup when putting it on a new Mac.
For contacts, I just used the sync feature to sync all gmail contacts to my mac, then signed in to iCloud with my me.com ID and synced to iCloud. That also worked like a dream.
I still use Google Docs/Drive for keeping my files/documents in the cloud. Apple has completely missed the boat so far with cloud files/documents and sharing.
I then reset my iPhone to use my me.com account. Fortunately, I only had a couple of inexpensive paid apps on my iphone and I just paid for them again with my me.com account, just to simplify things.
This proces worked easily for me only because I had been creating the backup music CDs from the early days of iTunes and creating an unprotected music library. But, maybe something in this process will be helpful to someone else.
Enjoy!