If it's not possible to gather the necessary information together to prepare your return, an extension requesting additional time to file will need to be filed. However, any additional tax beyond what has already been withheld or paid as "estimates" still needs to be paid by April 18th in order to avoid any late payment penalties plus interest. The extension, if granted, will allow up to 6 additional months (usually an October 15th due date) to file your return.
Finally, if you discovered that you forgot to: take another deduction, report a Form 1099 that you received late, or make other changes to your 2007 income tax return, you can still amend your 2007 return. The amended 2007 return needs to be filed by no later than 3 years after you filed your original return. Therefore, if you filed your original 2007 return before or on April 15, 2008, the amended return is due on April 18, 2011. If you filed your original 2007 return under extension on, say July 23, 2008, your amended 2007 return would be due by July 25, 2011. You get the extra days because July 23 is a Saturday and July 25 is the next business day.
As you can tell from the above, due dates are important and should be watched. I therefore always recommend that when you file a tax return with the IRS and/or your State's taxing authority, you should obtain proof for the date of filing. When filing by mail get from the Post Office a certified mail receipt with return receipt requested, and when e-filing get an e-mail advising that your return was received by the IRS or State's e-file center for e-filing.