You can develop Scala programs directly in MS Visual Studio. In the fall the Visual Studio plug-in for Scala will support the basic functionality expected of the IDE namely, code completion, code browsing, line breaks, expression breaks and exception reporting and so on. However, in the short term the current plug-in does not yet support code completion or code browsing. You can find a "how to use" guide here  and a slide presentation overview of Scala.Net project here .
The current implementation is already quite robust. The Scala.Net compiler does compile itself, some 100k lines or so of Scala code.
You can read about the latest developments (as of July 2011) by following this link . Scala on .Net is currently still work in progress.