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 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.