Thanks to all the misinformation that has been circulating since the 1920s, I don't believe anything I hear about Cuba. Especially if there was an article on a US-based news service like AP or Reuter's.
Interesting reason to keep this in the Microsoft area:
Nobody in Central or South America pays full price for Windows. You can go to any music/video store and find Microsoft Windows XP for $3. No shit. For this reason, many of the computers I used in Ecuador were using current software technology on creakily ancient hardware. For real, the Navy had a P2 running XP attached to a dot matrix tractor-feed printer from 1983. Down there, the hardware gets fixed. We rich (relatively) folks can afford to get a new computer if ours breaks, or get a new hard drive if ours flakes. Down there, they fix it, because a hard drive costs the same as it does here, while wages are 1/2-1/3 of ours.
Anyway, there's no reason to assume that Cuba was running legally licensed copies of anything, including Windows. So they were probably doing more to fuck Microsoft while they were using Windows than they will when using Linux. No American relations of any kind extend to Cuba, which definitely includes Microsoft's legal team trying to find illegal Windows installations.
However, PR speaking, this was a pretty slick move by Cuba. Making the simple choice of OS into a political issue was brilliant, and in non-US countries, this sort of statement will not go unnoticed. Anybody who says no to the US gets underdog respect, especially in South America. In many countries of the world, this will give Linux a new credibility, and poke major holes in Microsoft's foreign sales. Don't be surprised if Venezuela, Columbia or Malaysia start going Linux as well.
So, to sum up - this won't have any short term effect on Microsoft's finances. But the psychological damage to their foreign market strength could be huge.