The Terminal Services Team blog has a great explanatory post on the changes to remote administration using mstsc.exe with the release of RDC/RDP 6.1.
Bottom line: going forward, you won't be able to use the /console switch on mstsc.exe, you'll use /admin instead. The good news: you won't need /console. Read the post for a good explanation of all the behaviors given different client and TS version numbers, as well as why this change was made.
This article on redmondmag.com goes into why getting rid of /console access to session 0 is a good thing, and even puts forth the improvements to RDC as a compelling reason to upgrade to Windows Server 2008.