Yea, it's a pain in the arse.
Here:
Inside AdSense: Google Accounts landing soon
"Please note, however, that at this time it's not possible to update your login to an email address that is already being used as a login for AdWords or another AdSense account. We're working on fixing this, and you won't be required to migrate until we do."
Also, some other related reading:
» AdSense Force Migrating Everyone To Google Accounts » InsideGoogle » part of the Blog News Channel
Adsense and adwords cannot be used on the same account