Emre,
Will this always be the case? The SQL Server that most will have installed in is a crippled version which is limited to using only 1 or 2 gigs of ram which limits the performance of SQL server. Support for other DB's is desirable. MySQL, FireBird are excellent embedded DB choices which are easy to deploy and maintain. I have briefly browsed the code and it does not seem like a big leap to support other DB's if so needed. Is there a reason that MS SQL Server is the default. It does not look like any stored procedures or back end programmability is being used so there does not seem to be any dependency on MS SQL. Can you tell us more?
Abi