Newegg has listed prices for OEM copies of Windows 7 — technically intended for little computer building businesses, but there's nothing stopping you buying them. And as long as you understand some mostly-minor limitations, you can save some serious money.
Take a look at the table — that's not chump change. Newegg is also taking a further $US5-$15 off some prices for pre-orders before October 20.
The main two caveats: OEM product keys aren't transferable from one PC to another, and you'll need to install them on a formatted machine (usually the best way, anyway). Less of a concern is that you don't get any support or packaging; just the disc. Still sounds like a bargain to me. [Newegg via ComputerWorld via ArsTechnica]
Windows 7 Home Premium Professional Ultimate OEM $US110 $US140 $US190 Retail Upgrade $US120 $US200 $US220 Full Retail $US200 $US300 $US320 Full OEM (3 Pack) $US310 N/A $US550