This note is really an endorsement of Chumby rather than a Dash entry so I hope it survives the moderator's scrutiny.
Anyway, a major and important difference between the Chumby OS and SONY's closed system is that Chumby works abroad. SONY's Dash currently (June 2010) has a "missing XAPI service issue," which affects all customers attempting to use their Dash from a non-U.S. IP address.
As a tour of many non-SONY user bulletin boards will quickly show, dozens of customers in countries on practically every continent have purchased Dash recently and, unlike the Chumby, have been blocked from getting even the simplest functionality from this machine (even new system updates are stymied since it won't authorize). SONY marketing materials make no mention of this restriction and provide no warnings. The company has not responded to any of the questions raised in its own forum regarding the timing of a fix (short of other customers' suggestions of paying for a VPN terminating at a U.S. IP address).
This is prime territory for a class actions proceeding against SONY for negligent misrepresentation and its known product/service defect. Until SONY reacts to the XAPI issue, my personal recommendation is that no one purchase a SONY Dash if they plan to use it outside the U.S. or expect to travel with it. SONY has been unresponsive to customer queries and is grossly mismanaging its handling of this issue.
SONY's approach to encouraging development of apps is also quite pathetic to date.
I can only hope that SONY sees fit to be as open about its system as Chumby.