If you wait until Oct 26 for your Windows 8 upgrade, then you'll save $30. Right now, Amazon and other retailers and taking pre-orders for Windows 8. At $69.99, the upgrade may look like a really nice deal.
However, if you wait until Oct 26, you can download Windows 8 for $39.99 from Windows.com (through the Windows Upgrade Assistant). That's how I plan to get my upgrade.
All Windows 8 upgrade promotions will end on January 31st, 2013.
Also, remember that if you purchase (or have purchased) an eligible Windows 7 PC anytime between June 2, 2012 and January 31, 2013 you will be able to purchase an upgrade to Windows 8 Pro for only $14.99.