I don't think MS would do it, because there's no money in it for them.
The HD DVD addon was roughly $200 CDN.
If they started to sell an external bluray drive, the price of said drive, plus the 120gb 360 model, would have to be less than a ps3. The elite is 250 cdn, and I can't see them selling the addon for 50 bux.
to sell one at the usual microsoft jacked up peripheral prices, it would really show how much value the ps3 has built it, that microsoft charges for. PS3 has 120gb hard drive, bluray, built in wireless networking.
I think the games on the 360 are better, but the fact that sony doesn't nickel and dime you for extra parts earns brownie points.