<p>From what I've read, <a href="http://blogs.msdn.com/astebner/archive/2008/08/11/8849574.aspx" rel="nofollow noreferrer">VS 2008 SP1 and Team Foundation Server SP1 packages are traditional service packs that require you to first install the original versions before you will be able to install the SP</a>.</p>

<p>Is there a way, supported or not, to slipstream the install?</p>

Here's an
MSDN forum post
in which an MSFTie indicates it will be possible and that details are forthcoming.  Another poster is relaying results of her almost-successful attempt.  Looks like this will be doable soon.
Related:
how to slipstream Team Foundation Server 2008 SP1 (TFS 2008 SP1)