Just a quick note about
this interesting article from Dr. Dobbs, in which it's noted that SQL Server gained 2 percent market share last year, climbing from 13% to 15%. IBM and Oracle both dropped a small amount, from 22.4% to 22% and 48.9% to 48.6%, respectively. The open-source DBMSs (particularly, those that run on Linux) saw a huge increase in market share -- 47% overall -- but that is still not enough to put them anywhere near the top spots.
It will be interesting to watch those offerings in the next few years--especially given that MySQL has now apparently added some data integrity features, it might get taken slightly more seriously as a contender in the marketplace. Still, I think the open-source offerings need to take a hint from Microsoft if they really want to get ahead; I think the area in which they falter is clearly user interfaces and usability. Microsoft has done a very good job of making SQL Server incredibly usable, even to the point where non-technical personnel can often use parts of the product. I don't believe that the open-source DBMSs have figured that out quite yet.