This is worth discussing - if only to clarify what people mean when they say that Pathfinder is doing better than D&D.
For example - is anyone saying that the Pathfinder core rulebook has sold more copies than the D&D 4E PHB? Or, failing that, that it has sold more copies since release than the D&D 4E PHB did in the same time period? If that's the case (and someone has figures to show it - although that's unlikely) then I'll concede that Pathfinder is more successful.
The metric, surely, is "How many people are playing X, and will therefore buy future products?" Will Paizo be selling as many "Sourcebook #10" units in two years as WotC did?
One thing I find problematic with this data is that people are comparing the sales of brand new game with the sales of a 4-year old game. I have absolutely no trouble believing that upon launch a shiny new game's core rulebook can temporarily outsell a 4-year old gamer's 15th supplemental book - that sounds utterly feasible.
But the telling metric is how many people overall are playing each game once the new one has settled down and is no longer new. At that point, both companies will be selling supplemental product, and it will be interesting to see what happens then.
However, this is all a tangent to the original topic, which is the Open Letter and the GSL, and the benefit - or lack of - to WotC to make it easier for 3PPs. It's getting hard to keep track of the conversation, to be honest - too many tangents flying around now.
Well, I can give some data from the local Borders, but it is only for the local Borders. One thing that I can tell you has been painful for the local Borders has been returns - they have been returning a lot of 4e, and have since it was introduced.
Now, I am
not saying that the game was not selling well when it first came out, I am saying that the distribution chain was over supplying the local Borders - and was likely over supplying a large number of other stores as well. This does not make bookstore managers happy. They eventually get most or all of their money back for the returns, but the delay eats up profit.
This year past the local Borders returned the majority of the stock of Essentials they got in for Christmas, including nearly all the Red Box. (For what it is worth, I thought that Essentials in general, and the Red Box in particular, would do very well. I was dead wrong in that regard - the part that sold well was the boxes of tiles, which even Pathfinder players were picking up. And I thought that
those would be the poor sellers.)
They have yet to return
any Pathfinder - but they have been getting
cases of 4e Essentials and only half a dozen or so of the Pathfinder when getting initial stock (this is recent - until October they were getting in three copies of each Pathfinder hardcover, they have doubled their order). Then they have been returning most of each case, and selling through the Pathfinder. So they are seeing two things - more sales on Pathfinder, and that they are having 4e crammed down their throats by corporate.

So guess which product they are happier with?
Now they are getting fewer of each product for 4e, and have increased the amounts for Pathfinder.
They are also spining 4e and facing Pathfinder at eye level - so the Pathfinder is getting more visual impact on the shelves. Pathfinder has been selling more copies with fewer books than 4e across the line. The manager's view is that WotC has glutted their own market. So Pathfinder is getting exposure while D&D is spined on higher shelves. (Which is better than being spined on lower shelves - folks would rather reach than bend.)
However, he was happy that all he had to return for the Essentials books was their covers - the remaindered books were stripped and I believe the boxes were crushed.

Crushing and stripping games still strikes me as barbaric and criminally wasteful.
At no time under 2e and 3.X was any game selling better than D&D, not even when 3.5 was in its last gasps, nor when Vampire was at its peak. So, shiny new game or not, D&D not being the top RPG seller at the local Borders is new, and frankly that is a bad thing - I do not think that it is just a matter of Pathfinder selling phenomenally well, it is a matter of decreasing sales of D&D vs. competition, for the first time. After trying not to do so under 3.X, WotC has, for the first time, split their market.
However all the local Borders is stocking for Pathfinder is the hardcovers - I rely on a subscription for the Adventure Paths and special order any other softcovers that I want. If they were carrying the Adventure Paths then maybe they would see some returns for Pathfinder as well. But they aren't taking that chance.
There are dozens of books for 4e fighting for space, and only five for Pathfinder, and so far all the Pathfinder books have been of broad appeal, while some of the 4e books may be a mite focused.
Now, this is a sample size of one store, so not worth all that much, but I would still be surprised to find that it is far from typical.
Please, feel free to split this tangent off to a new thread, but I felt it worth mentioning.
On topic, I doubt that the letter will accomplish much, if anything. But then my respect for WotC has almost vanished, so I admit to personal bias. I loved the OGL, and thought that it was the best thing for RPGs that had come along in years. The GSL is more like a slap in the face with a dead fish. While I do not want D&D to fail I would not mind seeing 4e rot. And in my head 4e and D&D are separate things - the failure of 4e does not mean the failure of the game, merely of its most recent incarnation. (Which is, also in my opinion, blind optimism on my part - WotC is too invested in 4e for the failure of the line to be anything but a
bad thing.)
The Auld Grump, tired and getting overly garrulous.