The IRS has issued specific procedures and guidance for reporting Ponzi scheme losses. See Revenue Procedure 09-20 and Revenue Ruling 09-09 for the details.
In summary, it's considered a business theft loss; 75% (or such greater amount as the bankruptcy court will justify) of the "account balance" can be reported as a theft loss in 2008, which can produce a net operating loss (NOL) to the extent that it brings your business income less deductions, plus any positive non-business income less deductions, to less than 0. The remainder of the account balance can be deducted when the bankruptcy terminates, or a greater percentage than 75% can be determined to be reasonable.
Under a provision of the stimulus package, if it does produce a NOL, you can choose to carry the loss back to 2003, 2004, or 2005, in addition to the previously allowed choice of 2006. Any loss unused in the first carryback year can be carried forward (under NOL rules) until used up. See Publication 536 on net operating losses (not yet updated for the optional 3, 4, or 5 year carryback), and form 1045 and instructions (which have been updated.)