cschell,
Hmmmmmm.........you are very worried about what other folks are doing when there is a good chance the other products may never come out - In fact an Atari 800/5200 'cuttle cart' might be very attractive since both systems used the same core hardware (ok, the cart pinouts - 2 connectors - are differnt but is the storing and loading different?)
You already have a successful product launched so people would trust you. Waiting cause someone else is trying something or other folks who are hadny with a soldering iron could do it shouldnt stop you. There are many like me who arent good with soldering irons and chips that would like something like this.
Since the 7800 'ramcart' wont support 2600s there may be a market for such a device that does both - Dont let us hang in 'neutered 7800 only land', 2600/7800 would be great!