It almost sounds a little too much like those "Bill Gates will send you a check for forwarding this email" scams, but apparently Microsoft is subsidizing certain hardware purchases to encourage use of its Live web search. The "Live Search cashback" logo appears next to certain sponsored results for searches such as "cheap netbook" or "cheap wii"; log in to your Live.com account and you can find discounts of up to 30-percent at collaborating retailers.
Brian Jepson of Hackszine got an MSI Wind for $285, after a $50 MSI rebate and $144 cash-back from Microsoft (the netbook should have been $479). Several eBay sellers are participating, and you can find more details from the online auction site here. Discounts can't be made on express eBay purchases, automobile or real estate buys, or through payment systems from other providers, among other things.
Microsoft will give up to $2,500 cash-back in all, and up to six purchases under the scheme can be made through eBay. It's limited to buyers in the US, unfortunately.