The setup is: An MSSQL within a virtual machine, a Symfony 1.4 installation on Ubuntu with PHP 5.3.5.
In order to get this working, php5-sybase needs to installed and the DSN needs to be set correctly. In the PHP manuals, it says something like "mssql..", but this caused Symfony to throw the error "Couldn't locate driver named mssql". I used "dblib.." instead and it works.
One problem occurred when using Admin Generator on an existing table: 500 | Internal Server Error | Doctrine_Connection_Exception OFFSET cannot be used in MSSQL without ORDER BY due to emulation reasons." This can be solved by doing this.
Another issue was "SQLSTATE[HY000]: General error: 207 General SQL Server error: Check messages from the SQL Server  (severity 16)". This only occurs for tables with many (>200) columns. I removed unnecessary columns from the schema.yml and the problem was gone. Interestingly, the query with all columns was correct and worked without a problem copy-pasting it to SQL Server Management Studio. Thus, the MSSQL adapter seems to have a problem with textually long queries.
One last issue was the date format MSSQL delivers: It causes the sfDateFormatter to throw "sfDateFormat.class.php on line 487 undefined offset", mentioning the year when trying to format a month. I decided for a workaround and left out datetimes here as well. But I would be interested, how MSSQL datetimes can be accessed...
EDIT: I needed some of the previously unnecessary columns later and found, that the amount of columns was not the problem. There was one column with a German umlaut causing the problem.