How many 15x15s are there?

[rhu]

From the "Puzzle Corner" of MIT's Technology Review:

William Tucker’s goal in life (I suppose) is to solve every possible crossword puzzle. Step one is to determine how many there are. He is looking for 15-by-15 grids of “open” and “blacked-out” squares obeying the standard 180° rotational symmetry, with no words of length one or two (all open squares are contained in a row or column of at least three open squares). No row or column can be completely blacked out.

The answer can be found on the second page of http://www.technologyreview.com/files/26052/May09PuzzleCorner.pdf

I'll note that the solution does not take into account the maximum word count or maximum blocks rules that most editors enforce, nor do I think it enforces complete connectivity. But it's interesting nonetheless.