What is the indication that a phone is rooted? How would Market application know that in the first place?

I talked to developers of a board similar to Gumstix. Even if you get the engineering work for free, you still need to produce several test samples before your design is production-ready. And that's a very expensive part because of the circuit density. The total budget to build a final product and to launch a first batch of few hundred units is between $50K and $100K. That's why Gumstix is $200+. David Braben may, however, find some charity sponsoring which would compensate the initial costs. Then $25 would compensate only the manufacturing cost, and that may become a reality.