If you have a unlocked Windows Phone with root access, and you wish to use one Live ID for app purchases (maybe to get lower US prices) while using another for everything else, a homebrew solution has just popped up on XDA-Developers.
ID Switch lets you switch the primary live id on the phone and then buy the app by making registry changes and then rebooting the phone. When the phone turns on again, it uses the id provided by the user and treats this as primary id. To make purchases you still need the password for that ID however.
The XAP, for unlocked and rooted devices, can be found at XDA-Developers here.