[wcoenen]

You can do this on http://bitfinex.com.

But be careful about how much you trust any bitcoin service. Bitcoinica, another margin trading platform, was wildly popular but existed for only 6 months; they had to shut down after several hacks and people lost funds.

Also, the main way to get non-bitcoin funds into these kinds of services is via mtgox redeemable codes, but those won't be available anymore after the 10th of April because of legal concerns.