You need to have at least 8.0.6 installed.
Start a command prompt
open "rdp://full address=s:10.10.10.10:3389&screen mode id=i:1&Desktopheight=i:1024&Desktopwidth=i:1280"
I am still trying to workout how to pass a username with a domain, when I put in &username=s:localhost\administrator it appears in the login box as
For a full list of commands that should work see http://technet.microsoft.com/en-us/library/dn690096.aspx