Bah. Check out Hewlett & Packard's goals when they started a company.
http://en.wikipedia.org/wiki/Hewlett-PackardFred Wilson famously says that about 2/3 of the successes in his portfolio radically change when business they were in between funding and exit. In short, it's great not to get too attached to an idea.
I don't think these guys are being stealthy because they are defending their brilliant ideas (almost always a stupid reason to be stealthy), but because they are waiting for their ideas to gel into cohesive vision.
(disclosure: I know both of the founders but have no clue what they are working on!)