A Gartner/IDC report summarising the state of the database market in 2005 contains some interesting nuggets of information.
The database market is still growing at 9.4% (which surprised me a little).
OpenSource databases account for less than 1% of the market but are growing fast (47%).
The Linux platform (thanks mainly to Oracle) is showing the strongest growth (84%).
Despite these two statements of fact, Oracle are not perturbed by the threat of OpenSource (pass the salt cellar).
Market share: