The ZMan:What does that mean - everyone can play in the community games space... seems like an odd thing to put.
All CG submissions must be in XNA3. All DBP submissions are in XNA2. It sounds to me like they're saying they'll allow certain DBP entrants to submit an XNA2 build to CG, which is a nice little special consideration.
I'm guessing they're concerned about the number of submissions for CG launch, and want to have a solid lineup, so they're looking at the general high quality of DBP submissions (many of these games look great) and mitigating risk by hand-selecting a few of the better ones. Another possibility: up to ten semi-finalists may be offered XBLA contracts, but they have more than ten games in the running that look saleable, so they want an ability to expand their support for these games.
In any case, I think the motivation for this probably comes from the sheer volume of high-quality games submitted to DBP this year. There are a lot of games in the gallery that I'd buy if they were up on the marketplace, and I don't think that's escaped anyone's attention.