As you may have noticed, it's a little hard in 2012 to change your default workspace after first launch. OK, it's impossible from the CUI.
The solution? Startup switches! Take a look at this Technical Solution on startup switches, particularly the /w options: http://usa.autodesk.com/getdoc/id=TS22957 . Editing the Target section of the properties of my desktop icon for Map 3D 2012 from:
"C:\Program Files\Autodesk\AutoCAD Map 3D 2012\acad.exe"
"C:\Program Files\Autodesk\AutoCAD Map 3D 2012\acad.exe" /w "2D Drafting Workspace"
forces Map to launch in my chosen workspace. You can always switch manually after launching as well.