I think if said site do have them available (and they're not just trying to drum up interest - they are, after all, a very large retailer of Samsung monitors) they are holding them until Samsung give them the 'go'. As far as I can tell Samsung are planning on releasing the 350, 550, 650, 750, 950 and possibly 850 at around the same time. They are covering a lot of ground across their new series' monitors but they aren't giving anything away regarding release date (dare I say they don't actually know themselves quite how the release is going to pan out!)
At the moment I would speculate May at the earliest or possibly June - they were originally planning a 'Spring' release date but there have may have been some set-backs.