Absolutely - you may file any time!
If you do not owe any taxes - there is no late filing penalty and you may file any time and get your refund. You generally have three years after the return was due to claim a refund.
The sooner you file the sooner you can receive your stimulus payment. If you have obtained a valid six-month extension to file or if you are filing to establish your eligibility for the stimulus payment, filing by Oct. 15, 2008 means the IRS can process your return and issue a stimulus payment before the end of the year.