Yeah, similar ideas have been kicked around (mostly on the hardware forum) before. Compactflash is similar to IDE, and I remember someone talking about putting CF cards directly on the bus, which sounded interesting. I don't know if that's possible though.

Actually, it probably wouldn't be terribly complex to build a device that does this stuff, it's just that you'd have to limit it to a specific mapper (which really takes the appeal of it away I guess, heheh).

I just heard from someone who's making a hard drive interface for Coleco. It's a pretty simple system compared to NES, but it looks like it still needs programmable logic.
http://www.widel.com/