I think the question is slightly more interesting than you think, although the answer is probably still "not possible"!

If you are interested in running a fast database, then you want something that runs "natively"; this excludes all kinds of emulation, conversion, virtualisation, etc. The answer here is to use an RDBMS which is designed to run on Linux. So that means no Microsoft SQL Server.

The WINE project attempts to run Windows applications on Linux. There have been attempts to run MSDE and SQL Server Express:

http://appdb.winehq.org/appbrowse.php?catId=25

although not with much success. There are other similar projects running.

Don't forget you have the virtualisation option, where you use VMware, Xen, etc on Linux, and run a virtualised Windows OS. This is not exactly running MSSQL on Linux, but it is close and probably the simplest option.

Andy