Considering the economic situation (and around world, not just in the US) those are still some pretty impressive numbers. Summer is when kids are out of school, families are taking vacations, and teens are working part time (or full time) jobs, all of which detracts from gaming. This is also of course irrespective of the fact that very few new titles (decent ones anyway0 have been released lately for any platform.
The argument about PS3 sales are that if Sony drops the price, sales will go up. Sony has been losing money on every PS3 version sold to date, but the bleeding is not as bad these days as production costs fall closer in line - at least that's what I understand anyway. Sony is also still failing to produce a lot of quality titles. They have a few killer proprietary games like Little Big Planet, the Motorstorm series, Warhawk, and Metal Gear, but that's just not going to increase sales.
Top that off with the fact that companies like Valve are thumbing their noses at the PS3, and you have more headaches for Sony. As a PS3 owner however, I don't regret the purchase. Only about 25% of my gaming is on it anyway, and any serious shooters like Crysis or L4D or racing sims like GTR2 go straight to the PC.
Microsoft just has better marketing and exposure as well as of course lots and lots of games. Hell when Halo 3 was all trumped up two years ago, it made me want to get an Xbox just for that game alone - apparently lots of people did. But having just bought the PS3 a few months before, that wasn't going to happen.