Shorting & trading physical t-notes outright is pretty hard. You can be long or short t-notes but I believe to trade the actual note requires to buy or sell 100,000 dollars worth of notes at a time and the margin requirement is subject to the t-50 regulation( need to put up 50% margin).
Most banks, hedgers, and speculators trade the t-note futures at the Chicago Board of Trade. The margin is more favorable (1.5%). HOWEVER, this makes it more volatile...and shorting is as easy as going long. Plus, you can trade options on t-note futures at the CBOT.
Here's some lnks for info:
www.cbot.com
www.investinginbonds.com
www.thebondmarket.com