The first reviews are scheduled for the Tuesday, 17th of April but the samples are out as we speak. A lot of Nvidia partners already got the Geforce 8600 GTS cards and starting to distribute them under the veil of NDA. Officially there won't be any reviews till that day but you never know who might get the card without a signature.
ATI will come roughly a month later at least that will be the date when you see its answer to a mainstream market in the stores.
Nvidia is quite happy about its 80 nanometre parts and it ruled this market for years. We will be very surprised if ATI steals Nvidia's mojo but ATI's sub $200 is this time at 65 nanometre and will end up clocked faster.
Graphzilla is ready, and DAAMIT partners are soon to follow for this new fight.