Wow, I'm surprised to see so much hate on this thing considering how pro backwards compatibility the comments usually are here.
I'll by one if it's less than $100 for my slim.
There's so many advantages to this over using an actual PS2. Higher res, faster load times, HDD saves, using less/better connections on the TV, decent rechargeable wireless controllers and fewer lying around, access to PSN messaging (theoretically, I don't think you could access the XMB from PS2 games on the old systems), save space on the TV stand, less electricity usage (PS3 and PS2 both draw power when "off"), possibility of PS2 games for download on PSN, not having to worry about multiple optical audio cables (I already have 3 between my PS3, 360, and HTPC).
And for those claiming they're selling something that used to be "free." It wasn't free, backwards compatible PS3s were at least $100 more than the similar model without. And now the PS3 slim has more space, uses less power, and is half the price. You can hardly say it was free.
I've been saying they should so something like this for years, though my original suggestion was having designed the old fats with a slot for an internal component to do it which they obviously didn't do. I'm not sure how they would do this though, I don't think USB is any where near fast enough for this nor does it provide enough power for a external CPU/GPU combo. I hope this thing is being planned for actual release, but I question if it's for current models of the PS3 given the limitations of USB. Maybe it uses multiple USB ports or uses external power and handles 100% of the processing on the external box and only streams video over the USB back to the PS3. If OnLive can do that over the interwebs, Sony could easily do it over USB. So that's where my money is.