Matrox has had an adapter out for a long time that would allow you to do that with any video card.
OK, lets put it this way:
You can pay $300 for an HD5850
OR
You can pay $300 for a Triplehead2Go Digital module.
That's right, same price, only you don't get a faster graphics card.
NOT ONLY THAT!
But... with one, you get three outputs that are limited to a maximum of 1680x1050, with the other, that is the same price, and comes with a new graphics card, you can use higher resolutions!
Triplehead2Go was always a gimmick. It was always overpriced, it's horribly limited in terms of what it offers (3x1680x1050 or 2x1920x1200).
While the end function is the same, almost, in that you can use three monitors as a single display, one actually gives you graphics power, and support for high resolution monitors, while the other is horribly crippled.
What Matrox offers (support for 22" monitors, or running monitors at non-native resolution) and what AMD/ATI currently offer aren't really in the same league.