One of the most common asks for SQL Server related to servicing/patching (after uninstall of updates/SPs) is the production of slimstreamed media, ie where the Update/SP and the RTM are included together in a single plackage. Now there are a couple of reasons to do this, the first is to save time and complexity for the person doing the installs, the second is to allow issues in setup to be addressed. Well as part of the re-architecture of SQL 2008 setup the later scenario was enabled in a clear but perhaps unintuitive way.

If you snag the first Update for 2008 one of things it can do is put down updated setup files, that can then we be used by the rtm installer...

Bob Ward from SQL CSS has a great post that explains it in detail here.