Harry, I believe a significant portion of the tools are written in Java, so I would imagine what you are asking should be technically feasible as soon as the Java SDK is available. Whether this would be practical or supported in any way by RIM is a different question.
Of course, as was pointed out, using the virtual keyboard for any significant work could be a fairly unproductive way to work. There are little Bluetooth keyboards, though, which may help with that issue for anyone hardcore enough to want to continue development on the tablet itself.
Being able to tweak things or try small experiments certainly isn't a bad idea.
I've already got my eye on the possibility of using Python, possibly through a layer that can talk to the AIR stuff (or bypassing it and going more native). The full package is already "available" in the simulator, and with Python the compiler and batteries are included... whether it will be in the next or the final releases is of course an open question at this point, to all but insiders.
