I'm not talking about a continuous build process, but the excellent feature in Eclipse whereby any java code changes are automatically built in a background thread. With this enabled, running unit tests or applications is immediate. Going back to the 'manual' builds of VS20XX is quite painful.
I've only seen this working in Eclipse for java code - does anything like this exist for .NET languages, in any IDE?
You could use msbuild in some background process, periodically building your solution, but this would yield to file locking issues and generally would be a crappy solution. Why not set up say CruiseControl.NET? it's not complicated, doesn't need any source control (could use filesystem) and basic setup for a project is very simple.
Check out Cruise Control: http://cruisecontrol.sourceforge.net/ It does exactly what you are asking for.
Actually, in Visual Studio - one of the best features in Visual Basic that is not in C# is that Visual Basic is compiled in the background as you work. Though, I'm not sure if it speeds up your unit tests, etc.
You might look into using TestDriven.NET, you can right click and execute a single unit test very quickly in most cases, and it will only compile what is needed to execute the unit test.