You can slipstream SP1 yourself, but it turns out there's an easier way. First, install the trial version of MOSS 2007 with SP1 (32 bit or 64 bit). After you install the trial version, upgrade from the trial version.
- In Central Administration, on the top link bar, click Operations.
- On the Operations page, in the Upgrade and Migration section, click Convert license type.
- On the Convert License Type page, in the Enter the Product Key box, type the new product key.
Thanks to Kirk Allen for the tip!