Don't do that. Switching workspaces is a slow, heavy, operation.
The "design decision" in Eclipse is that each developer has one workspace, which contains all of their projects. The feature that's intended for switching between groups of related projects is Working Sets, perhaps in combination with Perspectives if the various working sets require different tool layouts.
The only good reason I can think of for using multiple workspaces would be if different projects required a totally different version or configuration of Eclipse. Then, yes, you'd probably need a seperate workspace for each installation. (But you wouldn't switch between them in the same copy of Eclipse!)
Most Eclipse settings are stored in the .metadata folder within the workspace root. If I had to use multiple workspaces for some reason, I'd probably create an ant script or batch file (or shell script, depending on platform), that copies my .metadata file from my "main" workspace into the others, and hook it up to the build menu in Eclipse. You could even create a custom launch for Eclipse that copies the .metadata into a workspace, then launches Eclipse pointed at that Workspace.