Gmail "tags" each message, for the "Inbox view" based on account. I'm not sure if this is done on the server or on the client side, but again, the Droid's app functions this way.
It wouldn't take long to setup on gmail (web account settings), and test with the 900.
I dont believe i can do this with my work account... I cant forward my work exchange email to gmail.... That is really why i say universal inbox is best, i shouldnt have to go through gmail to do it.