Had an annoying issue today where the customer and I tried to install a primary server but kept getting stuck with the option to install a primary server greyed out. Turns out we were using the SP2 upgrade media that was used when they were upgrading the SCCM sites in their environment awhile back.
So in short, make sure you have the slipstreamed SCCM 2007 w/SP2 media instead of upgrade media to prevent this bone-headed move :)