The installer for Visual Studio 2008 SP1 is a tiny file, just half a megabyte.  It is a just a bootstrap application which downloads the actual files required to perform the update.  We have a bunch of development machines here in the office at Straylight Studios, so we don't want to have the bootstrap installer downloading all of the content for each and every machine.

If you want to download the entire service pack content then you can call the installer with a /CreateLayout commandline flag, and the folder where you want to download the files to.  I think the full contents are over 800Mb so the download will take some time.

VS90sp1-KB945140-ENU.exe /createlayout \\server\share

UPDATE:
I've also just found an ISO image version of the service pack.  This may be simpler for everyone