I didn't stick around to see this, but SP1 was on pace to take 5 hours to complete the install. Then overnight it apparently decided the install failed (finally) and undid the install. So my machine was already booted into RTM this morning and works fine.
Good work, SP1 team (sort of). I only had to get past a BSOD to then complete an excruciatingly long install to then have it uninstall... but it at least left me with a working (non-updated) machine.