just for reference, if you're trying this, the command would looking something like this:
at HH:MM /interactive cmd.exe
right? looks right. this will spawn a cmd shell alright. you'll see it in the processes. you won't be able to access it. why is this?
it turns out that the interactive switch is indeed true. you have to be logged on at the console. rdp, terminal services, etc... they won't show it. good news is, if it's 2003, you can connect to the console session using this command:
mstsc /v:[servername] /console