[scainburger]

This DOES work with the Steam version!
I have done it!

PLEASEPLEASEPLEASE MAKE A BACKUP OF YOUR GTA FOLDER
PLEASEPLEASEPLEASE MAKE A BACKUP OF YOUR GTA FOLDER
PLEASEPLEASEPLEASE MAKE A BACKUP OF YOUR GTA FOLDER
Also make a backup of your user files in your documents folder.
I have no idea if this will work, I am assuming it will but no guarantees it won't screw everything up.

I can't remember where I found how to do it, so I'll give this way a go:
Rename your "gta_sa.exe" to something else (for backup)
Download http://dl.dropbox.com/u/8607449/GTA_..._downgrade.zip and extract to your GTA SA folder
Done! You now have version 1.0 and can install SAMP!

Hope it works