I remember discussing this recently but can't find the post. There is a way to do it. Here's the instructions I had from Apple. Not tried it yet but keen to do so. If you do try it out let me know if you were successful or not.

The only problem I can see is if you start using the account where you didn't purchase the items you won't get the history in the app store on your iPhone or iPad.

I understand that you have multiple iTunes Store accounts which you'd like to consolidate into one. I'd be glad to address your concern.

Just a heads-up, it is not possible to merge two or more iTunes Store accounts together, which means you won't be able to transfer over your purchases from the iTunes Store account "XXXXXXX" to "XXXXX@XXX.com". However, you can play all of the titles you've purchased across multiple accounts if you authorize your computer for each account. iTunes Plus purchases don't require authorization so you won't have to do anything to be able to play those or transfer them to iOS devices.

If you have any purchases that require authorization, you can follow these steps:

1) Make sure you have the latest version of iTunes. You can download it free of charge here:

http://www.apple.com/itunes/download

2) Open iTunes.

3) From the Store pull-down menu at the top, choose Authorize This Computer.

4) Enter one of your account names and the password for the account. If you have forgotten the password, you can reset it here:

My Apple ID
https://appleid.apple.com

5) Click Authorize.

Repeat steps 3 to 5 for each of your accounts.

Yeah, I have to do that occasionally because I bought some stuff from iTunes using an Apple ID I no longer use (I don't have access to the email address it's tied to any more). It's not really hassle...

It was on twitter, but thanks very much for that caz.

No wonder I couldn't find it