A few of us were working just this morning on trying to prevent this confusion in the next version, so it seems like a decent time for a blog post.
When the TFS 2008 (and 2005 for that matter) setup is asking for "Reporting Services Data Source Account", it's not asking for the account the Reporting Services itself is running as. TFS doesn't need to know (and doesn't care) what account RS itself is running as. Most of the time if you accept the defaults it'll be Network Service, but TFS really doesn't care.
Going top-down on the explanation:
The field "Reporting Services Data Source Account" is asking for the account/password you want to use for the credentials inside these 2 data sources we're going to create in Reporting Services.
To reiterate, it's 100% unrelated to whatever account Reporting Services itself is running as. We (TFS) don't know and don't care what RS is running as :)