Are you looking on the manage domains screen? All the way on the bottom left hand navigation on the dashboard there's Upgrades -> Domains.
Going there should give you two domains, your custom one and the original one ["WHATEVER.wordpress.com"]. Click the little button next to it and hit 'update primary domain'.
Then you should be able to map it to another one.
Keep in mind, however, that the cost for the domain upgrade is on a per blog basis, so if you've paid for the upgrade $10/year for blog A you will have to pay the same amount again for blog B.
If you've mapped the domain accidently though, it might be worth contacting support and see if they change the subscription over. http://support.wordpress.com/
Hope this helps.