As i said with the January Cumm Update, It works just for the email.
But if you use someother update or with SP2 or Aug Update you will be able to populate Domain\user name or just UserName or email address.
If you are using MOSS SP1 with January Update and have trouble populating the People field you can achieve that by Managed Code or a code-less method would be add the User Information List from the RootWeb or the CurrentWeb as a Data Source and use the rules to get the email of the selected user (AccountID)
Rule:
Create a datafield called Email with string in the Main data source and set the rule on the AccountID as
Condition:
When Accound is not blank
Email=@email(@login=AccountID) where email and login are fields from the Secondary data source and the follow the usual workflow procedure.