You have until at least April 15th, 2010 to amend the 2006 form 1040. It is three years from the filing deadline...not the date filed.
If you want more specifics, then:
The IRS generally gives taxpayers three years after the original return's filing date to make any changes with a Form 1040X. If you filed early, you get three years from the return's due date to correct any errors.
Your window to amend closes a bit if you didn't pay all the tax you owed when you filed. In this case, you must revise your return within two years of the day that you finally paid your full bill to Uncle Sam. If, however, the two-years-since-payment date arrives after the standard three-year time limit, the IRS says you can amend your return using the deadline that comes later. Similarly, if you paid your taxes late, but not that late, and the three-year grace period provides you more revision time, you can use it. -- care of Bankrate.com
(Extra: I have no idea why I got a thumbs down. What is said is 100% truth. I learned it during my Masters in Tax degree at Villanova. I suppose some yahoo (no pun intended) wants to disagree. If you paid mortgage interest, you are referring to a 1098. You must file (letter must be postmarked) the ameded return by the deadline I mention above. If you file it one day late, you will not get anything back from the IRS.)