I had a friend who had the only copy of his marriage licence on his PC. Somehow he deleted the folder off his desktop and then emptied his recycling bin. Turns out, even with that, drives dont actually erase the code, just free's up the resources for the data space, basically until new data is written to that exact physical spot on the disk, the data can be recovered. This is the case even on reinstalling windows using the fast format.
Long story short, as soon as he realized what he did, he took it to a computer shop that specializes in data recovery. They were able to completely recover the deleted data. I dont know what the cost was, but for some people, a single file is worth lots, with GT5 saves, some people put hundreds of hours into a save.
Far as I know, data recovery can be done on a PS3 HDD so long as its not a SSD.