Just a short anecdote...
I was playing Marathon Infiniti today, and I was getting a little tired of it... so I decided to fire up M1A1 instead. I put AlephOne in the proper folder, loaded the proper files... but when I opened my saved game file, it said it was from a "different scenario," but would open anyway. I was a bit confused... but I figured it out when it actually opened up. I checked the map, and it said "Naw Man He's Close." That would be the Infiniti map I was just on... I had opened up my Infiniti save file in M1A1!
The result was quite amusing. The map was right, and the level played just like in Mi, but the textures were all M1 textures, and the weapons and VacBOBs were especially funny-looking. The VacBOB was just HALF of a BOB, so it looked like a torso and head only, crawling on the ground.
I'm probably not the first one to do this, though... or am I? Any experiences?