Some sites seems to be citing April 13th as the release date for the NV40. Here's one that is cited as the source for this, but I'm not quite sure if that means what they think it does.

Just because someone will get cards based on nvidia's next-gen gpu, doesn't mean they'll get them then and there, but rather that they will get them when they're available.

Anyways, the date for the R420 seems a bit more certain,
Here is a press-release from ATI about their attendence of the GDC, from 22 to 26 of march.

There's also been nods and semi-cryptic comments that this will be the event that the R420 will be launched at.

Yes! Muhahahaha I can't wait!