No, this is not a way to get your investment losses back.
But there are two situations that at least allow you to make some lemonade of your lemons. Caution: You should consult with your tax adviser before acting, a mistake could cost you big time.
You made a 2007 contribution to an IRA account. The value of your IRA account has decreased since then.
The law allows you to withdraw your contribution plus earnings from your IRA account up to the due date of your tax return (including extensions). If the value of your account has gone down since you made the contribution, your earnings will be a negative number. That's OK. For example, if you put $4000 into your IRA account, but the value of your account has shrunk by 25% since then, you can take out $3000 and it will be treated as if you had never made the original contribution.
After you take out your original contribution, you are again free to contribute the maximum amount for 2007. So, while you will not get any losses back, you get the opportunity to add more money to your IRA account (and hopefully make better investment choices this time).
Points to remember: For this to work this value of your ENTIRE account must have gone down. It does not matter if the value of the specific investments you bought with your 2007 contribution went up or down. You do not have to redeem the specific investment you made with your 2007 contribution, but you may redeem any investment that is in the same account. The deadline for making a 2007 contribution is April 15, 2008 -- no extensions are allowed.
If you want to withdraw your 2007 contribution plus earnings, please be sure to inform your IRA custodian that you are taking a corrective distribution so that they code the distribution properly in reports sent to the IRS. You should ask the custodian to calculate the earnings for you, but if they refuse, use Worksheet 1-4 in Publication 590.
You converted a part of your Traditional IRA into a Roth IRA in 2007, but the Roth IRA is now worth less.
Say you converted $10,000 from your Traditional IRA (TIRA) to a Roth IRA. But the Roth IRA is now worth only $5000. You still have to pay taxes on the full $10,000 you converted (assuming it was all before-tax). It just doesn't seem fair, does it?
You can use what is called a "recharacterization" to unconvert that money and put it right back into a Traditional IRA account and you can forget about paying any taxes on that conversion you did in 2007.
You have to recharacterize the converted amount plus earnings. Again, the earnings can be a negative number. Earnings are calculated using the change in value of the ENTIRE Roth IRA account in which the converted funds were placed. So it does not matter whether the specific investments you bought with the converted funds went up or down.
You have until the due date of your return (plus extensions) to recharacterize any conversion you performed during calendar year 2007. If you file your 2007 return by April 15, 2008, you get an automatic extension until October 15, 2008. Otherwise you need to file Form 4868 by April 15 to get an extension. So even if the value of your account hasn't gone down yet, if it goes down by October 15th, you can still uncovert and file an amended return to get your money back.
You can reconvert after waiting 30 days, but the reconversion will be reported on your 2008 return. It is too late now to do a conversion or reconversion for 2007.
Again, make sure that both the custodians of the Roth account and the TIRA account know that you are performing a recharacterization so that the transaction will be coded properly. Ask the custodian of the Roth account to calculate the earnings. If they refuse, use Worksheet 1-3 in Publication 590.
Recommended Reading
"Contributions Returned Before Date of Return." Publication 590, page 33.
"Recharacterizations." Publication 590, page 30.
"Recharacterizations" Form 8606 Instructions, page 3.
"Return of IRA Contribtions" Form 8606 Instructions, page 4.