A SATA2 drive works fine in a SATA3 system. Also, the 4-pin power connector is a molex plug and not SATA related. The MBP connector has separate power and data. I don't know what kind of connector you have, but try formatting it as an external drive using a USB adapter before installing it.
Can you get a decent price for it? It may be better to sell it and buy a proper drive, especially since OSX doesn't support TRIM and Intel drives depend heavily on it.
I have it up for sale already and it won't be long. However, TRIM is not only fully supported in Lion, which I've been using exclusively since the update for Dev. Preview 4 was released, but TRIM is supposedly also enabled on the specific build of Snow Leopard that's on the recovery disks that come with the 2011 MBP's. I'll try to find where I read that and yes, it sounded strange to me, too, but even if that info is wrong, TRIM can be enabled in Snow Leopard. I've seen screen shots of System Profiler under the SATA option.
Co-incidentally, after taking it back out and put the stock (750GB HDD) drive back in, I did exactly what you had suggested already and that was putting it in a USB 2.0 enclosure to see if the MBP would at least acknowledge its existence and sure enough, it did! It was strange, though, because OS X treated it like an additional partition of the stock drive that came with my MBP. I did format it "Mac OS X Extended (Journaled)" in Disk Utility. In fact, I've only ever used Disk Utility on it, just in case that matters.
What should I do now? I guess the only thing I can do is to try to put it back into the MBP and see if I can get it to work now that it has been formatted "correctly". What say you, sir?
Anybody else want to chime in is more than welcome, too.