It's 15 October POSTMARK. Alternatively, you can courier to the IRS office at the US Embassy. If you use a courier, you should use an IRS approved courier. The approved couriers are UPS, Fedex, and DHL and maybe one other (I couldn't find the list yesterday).
If your local post office is open, bring it in person and WATCH the man postmark it. Send it AirSure and you'll get proof of posting.
Alternatively, you can ask for a further extension in a letter addressed to the same address that your tax return should be posted to. This further extension will give you until 15 December 2007. Unfortunately, you will still have to ensure that it is postmarked by 15 Oct 2007.