FYI: The MS JDBC Driver is completely tested by IBM for use with WebSphere App Server v6.1 and v7.0.
From the IBM Announcement for WebSphere Application Server v7.0:
WebSphere Application Server V7.0 does not include the WebSphere Connect JDBC driver that had been provided in WebSphere Application Server V6.1. To replace the functionality provided by the WebSphere Connect JDBC driver, WebSphere Application Server V7.0 is tested with the DataDirect Connect for JDBC driver and the Microsoft SQL Server JDBC Driver, which may be obtained from DataDirect Technologies or Microsoft.
Connect your WebSphere/Java apps to SQL Server with this driver. Actually, you can connect *any* Java application to SQL this way - it doesn't have to be running in WebSphere. If you develop in Jetty or Tomcat or JBoss or Geronimo, the MS JDBC Driver will work fine for you.
If you have .NET applications, or Office apps, of course you can connect them to SQL Server also. In this way, SQL Server can serve as a nice interop point between Java and .NET applications, or between Java and MS Office (Let's say... an Excel spreadsheet).