Can I take a loss on conversion? If so, how do I do so (i.e. what U.S. tax form do I use)?
The rules are complex. See How to Deduct IRA Losses, at Smart Money. You must liquidate the entire account (bad) and the loss is subject to the amount exceeding 2% of your AGI. If you are subject to AMT, you may lose any or all of that remaining amount.