Not a flop. It expands the PSP market to sell to a pretty select set of consumers, and in that regard, its doing exactly as expected.
That 6K increase is actually a 120% increase, and outside of Japan (where the PSP Go will likely be the most attractive), that's pretty decent for a re-release of the same console, yet in a new for-factor, especially since those consoles will generate about 2x the normal profit margins on PSP games sold for it, for the publishers.
Sony couldn't lose with such a device, unless they produced more than they could ever sell. Since the Sony store itself has them on backorder, I'd have to say that its a success, without question, even if the other retailers were smart enough to stock up and meet demand, and didn't sell out on day 1.
Claiming it flopped, by comparing it to the DSi, is like claiming the latest model of Corvette is a flop, because it didn't sell as much as the new Civic.