Do you mean for playing PS2 games which can be purchased from the PlayStation Store?
If so then the basic mechanic (or engine) is probably the same in the code for each title but the developer or Sony has to tweak each PS2 game's code individually to ensure it runs correctly on the PS3
It's because of this tweaking requirement we don't have full, blanket PS2 software emulation on the PS3
I am not sure but i have found from experience PSone disk games seem to run better on a PS2 than a PS3. I have a PS2 and PS3 both connected to TV at the moment and prefer to use the PS2 for PSone games. Still got a couple of PSone's knocking about still the best way to play original PSone games, just don't want another console connected to my TV.
ps i read your question as your referring to original PSone emulation?
Does the PS3 use the same Playstation emulation software as the PS2?
Assuming you're talking about playing PSOne games, then I'd say No. I think the PS2 has some built in hardware that the PS3 lacks (well, maybe the very first PS3 models had some extra hardware, but most units don't).
Case in point: if I put my Final Fantasy VIII disc 1 in my PS2, it works without problems. In my (slim) PS3, it doesn't work - it doesn't even get to the opening credits. The PS3's emulation may get better over time, mind you.
Note that the PSOne classics you can download from the PlayStation Store (for PS3, PSP and Vita) have been patched to make sure they work - FFVIII included. (Though other emulation problems occasionally creep in, as we know...)