I'd say it has until the PS3 starts to pick up.
That's really the only answer. Developers are still releasing stuff for the PS2, and it's going to keep dwindling until either the PS3 takes off, or the PS2 stops selling. The question REALLY is, how long will GOOD developers keep supporting the PS2?
Here's the tricky thing, though... developers know now the the Wii and PS2 can release similar games, which may serve to increase the lifespan of the PS2. Let's say you've decided to ride the Wii wave and make a game for it... why not port it to the PS2, just because it's easy to do. Both have huge install bases, right? The Wii will definitely expand the life of the PS2, that's fairly certain.
Really, though, it's on Sony. They wouldn't even dream of still supporting the PS2 if the PS3 had sold like hotcakes (the Wii). I mean, they needed to make the PS3 cheaper, so what was the first thing to do? Backwards compatibility. It helped them because it lowered the price of the PS3, and helped to keep sales of the PS2 going, which they actually, you know, MAKE money on. If people truly made the switch from PS2 to PS3, PS2 (or at least quality games on it) would go the way of the XBox very quickly.
Anyway, I've rambled long enough... if you don't want to invest in a still backwards compatible PS3, it's certainly a reasonable idea to still pick up a PS2. This is, of course, as long as you're buying it for the BACK CATALOG. If you buy it for 'new' games, you'll be sorely disappointed, period. In that sense, PS2 is effectively over, barring the 'Wii ports' I mentioned earlier. In reality, they'll be nothing new that's truly compelling in any sense.