I always knew the red X's were there since release but never really thought of it, then I was told they were marking dragon mounds/treasure spots.

I recreated my first character from 11-11-11 and after about racking up 60 hours on him, I noticed those X's again and thought I would check them out. To my surprise after a search on Google, A lot of boards sill do not know what those marks truly mean. Some were saying they mark the birth stones but most of them are not even near a stone, as well as the spot having nothing of interest around it. What is more ironic is that the X west of Winterhold and the X north of Shor's Stone marks a birth stone right on the money.

So did I miss something or do we not know 100% what those red X's mean?

I always assumed that some birthstones were moved after the map was already completed, so they don't all match up.

But don't some in the water point to shipwrecks? (or maybe that's just the Morrowind map)

That was my first thought, too. I think the map we got was a "beta map" but the locations got moved. Makes sense since I do remember the paper map being revealed months before Skyrim released.

As for the shipwrecks, I don't think so but the X I mentioned that is west of Winterhold is really close to Pilgrim's Trench...

Is http://i.stack.imgur.com/YD87e.jpg you're talking about?

One of the markers west of Solitude must be Meridia's Shrine. Another, northwest of Dragon's Bridge, Hag's End or Volskygge. And another, in Lake Ilinalta, must be the Lady Stone.

I say they're various points of interest -mini spoilers in the form of "x marks the spot".

There's a spelling error on "Rorickstead". Funny.

My theory is also that they mark the birth stones and also some other important places. It is possible that the locations have been moved after the map was made. One way to explore this would be get to CK and see if the places that seems to have no point of interest have a named cell (other than "wilderness"). There are many locations that were left out, but the cell is still named in CK (like the mill in Kynesgrove).

I don't think so. The printed map doesn't exactly match it's ingame counterpart. It looks out of scale here and there. A nice touch, since they obviously wanted a medieval look on it -and maps of that time period, were far from accurate

They all are Points of Interest for the Adventurer, and do mark actual locations in the Game.

Some of the Markers are Standing Stones, some are Dragon Priest Lairs, Dwemer Ruins, and Daedric Sites.