Currently Being ModeratedOct 8, 2012 5:22 AM (in response to matnorton)
Well I should start by saying that it's a good thing you don't want to merge IDs, because there's still no way to do that. As for what you're proposing, you can do that. Just sign into your account in iTunes, download everything, sign out and sign into the other account and download everything. This should get all your current stuff on your computer.
You can then create a new Apple ID or use one of the current ones on the Mac without a problem.
As for the sort of local cloud that you're proposing, you can do that with iCloud, the problem is you'll need to have one Apple ID as the primary ID on all your devices, now you can use others, for iTunes and some other things if you wanted to. If you do that, you could have photos taken from any devices appear on your other devices, including your Mac, using Photostream, and iWork documents could be synced between devices. Mail messages will get synced between any devices set up with the same account.
Let me know if you need help getting something set up.