The on-lap "problem"
Surely even as is it works at least as well as when people put any other tablet and bluetooth keyboard into a 3rd party case?
However I think there is a better solution. If its detachable then why don't they make a second style of keyboard? Something thicker and more substantial of more netbook depth. That should allow room to include a slide out tray to support the kickstand (even better if it locked to it somehow) or perhaps a hinged support for the screen without using the kickstand. Either way there also ought to be room in such a keyboard for a large second battery.
If they did this it would both increase the flexibilty of the Surface while mitigating the compomises a little. Want to the sort of work you'd normally take a laptop for - use the proposed chunky keyboard. Need to slim down a little but want a cover during transport - use one of the existing keyboards, not really any chunkier than the covers for other tablets. Need to travel really light or just browsing in bed - leave both keyboards out of it.
True it still might not be as good a laptop as a dedicated one, but it'd be getting closer. It'd also push the price higher to buy two keyboards, but not compulsary to do so and you'd need to balance against not buying both a laptop and tablet.