USAFRet :
Personally, I despise mail in rebates. If they can afford to sell the part for $80, then do that.
Don't charge me $100 and then give me $20 back in two months.
The manufacturer usually has a contract to sell the product to the retailer at a certain wholesale price. If the manufacturer wants to lower the retail price of the item below that price, they would have to renegotiate that contract because the retailer is not going to sell the item for less than they paid the manufacturer for it (at least not until they're almost out of inventory and they want to get the last remaining units out of the computers).
Or the manufacturer can issue a rebate. The contract stays in place, the retailer sells the product at the original price, and the manufacturer takes care of making up the price difference with each buyer.
It's more complicated than this (manufacturer does not handle the rebate itself, it hires a rebate company to do it for them), but that's basically the main reason rebates exist. Rebates prevent the logistical nightmare of re-pricing a bunch of products which are already in or part-way through the supply chain.
They also prevent someone from buying up every item on sale and listing them on eBay at a higher price. When HP shuttered its WebOS division and had a fire sale on Touchpads for $100 each, I tried for 3 days to order one without success. Meanwhile several high-profile ebayers bragged about getting in orders for 1000 or 2000 units. With a rebate that's limited to x units per household (address), the manufacturer is doing its best to make sure the price discount goes into the pocket of the end user, not some middleman.
The Newegg and Staples rebates are very reliable, they just take some time. The rebate processor in El Paso, TX is also pretty good. Ask around for different rebate processor addresses - some are good, some are con artists. Read all the fine print to make sure you're submitting the rebate correctly. And make copies (PDF scans) of everything before you mail it in. Sometimes they do lose stuff and will ask you for proof of your submission.