Nordic Hardware believes the first 40nm GPUs from NVIDIA and AMD will arrive in the first quarter of 2009. Little is known about the AMD chip but the NVIDIA 40nm GPU will most likely not use a new architecture as the firm uses the same strategy as Intel, they don't introduce a new architecture and new use a new fab technology at the same time to avoid problems.
Very soon NVIDIA will roll out its first 55nm chips and there's a rumour that we might expect a DirectX 10.1 GPU with GDDR5 memory from NVIDIA in late Q4 2008 or early 2009.
We also know that NVIDIA is preparing to launch a 40nm chip as soon as TSMC get its act together and start pushing out half-node wafers. Right now it seems like this will happen in late Q1 2009. This is most likely not the same chip as NVIDIA doesn't do new nodes on the first date with a new high-end card. Things might have changed in the Santa Clara headquarters, we don't know that certain so take it with a pinch of salt.
AMD is expected to hit 40nm in Q1 2009 as well, even less is known about that right now.