I actually plan on using an iPad as my primary computer so have thought a great deal about "the keyboard thing."
For the home base, buying the keyboard dock is a no brainer. As for the OP's idea...ultimately it's all in the execution. If the accessory helps protect the iPad, adds minimal weight beyond the Apple BT keyboard, and doesn't scratch the screen when folded....then sure it might sell IF well executed.
HOWEVER, I think this also requires that two other questions be answered first:
1) How is the onscreen keyboard? I mean, when the iPhone was introduced (but not shipped) so many people thought it'd be useless. Now I'm so proficient with the iPhone's keyboard that I scoff at the idea of a hard keyboard on a phone (and I'm a former BB addict).
2) How portable is the keyboard dock itself? Does it fold? Will it have it's own separate case?
So, to the OP, I think your idea supposes that the answer to those questions is: 'bad' and 'not very.' Given Apple's history, I wouldn't count on it.
TM