There's not much reliable data out there to estimate this ourselves, and SpaceX isn't exactly forthcoming with details. Or perhaps they would be, but media aren't asking the right questions during press conferences. So far, the only somewhat reliable data point is the canceled recovery attempt during the DSCOVR launch on February 11, 2015;
According to SpaceX press release:
The drone ship was designed to operate in all but the most extreme
weather. We are experiencing just such weather in the Atlantic with
waves reaching up to three stories in height crashing over the decks.
Also, only three of the drone ship’s four engines are functioning,
making station-keeping in the face of such wave action extremely
This would suggest that SpaceX are confident they could land and keep the first stage stable on the drone ship as long as it can provide a stable surface to land on and smooth out the swelling of the sea during strong lower atmosphere winds.
Now for a bit of speculation on my part; Stability of the landed first stage, assuming weather permits it and all four of its landing legs are properly extended and on the platform, shouldn't be such a big deal even with occasional (not gale force) gusts of wind. Practically exhausted of propellants, with perhaps up to a few seconds of literally fumes of kerosene remaining (that's why it appears to explode so fast once the propellant tanks are breached on impact, or what Elon Musk called closer to a fast fire than an explosion), the first stage's center of gravity would be low. If position of the landing legs' mount points suggest where exactly, and those are quite low, it would have to be in the lower 1/3 of its length, possibly quite a bit lower, since lateral pressure on the truss increases with mount point's distance to CoG (law of the lever).
Sadly, I can't estimate this analytically, because ideal pressure angle isn't known. For structures that can withstand pressure equally both in lateral and long axis direction, that would be 45°. But rocket stages can't be built like that, so this angle would be substantially higher, in favor of pressure along the long axis (vertically). If we knew that angle at legs' mount point ring (that's got to be reinforced for lateral forces, but how much?), and exact point of CoG, and we already know rough total weight of near dry landed stage (it was mentioned during the CRS-6 post-launch news conference), stage's height, how much legs extend and how high they suspend the stage (can be estimated through photographs), we could extend stress line from CoG, at that given angle, towards the landing surface and compare that to stage's total height. Then, with its know weight and surface area, we could calculate wind shear it could withstand laterally before it tips over. Sadly, as we lack crucial data, not even such estimates could be done.
Anyway, I wanted to write this so we identify data that we still need to (roughly) analyze landed first stage's stability, and why with too many unknowns, it can't be done. I'll trust that SpaceX can do, and did do, extended analysis on (to them) known structural properties of their first stages and will take their word for it - that if the barge can handle it, landed first stage should too. But I wouldn't rule out possible future use of mobile anchor shoes to clamp landing legs in place with their weight, or some other means of achieving this as the barge heads back to land with recovered stage on it, or prior to refueling and flying it to shore, as SpaceX indicated they eventually intend on doing. Of course, ultimate goal is still landing on terra firma, once they demonstrate to FAA and other authorities that it's safe. But that's a different story.