Apple are not going to enter the $500 PC market for several reasons.
#1 Cheap PC users don't buy any software (except for Office, possibly), so the only money to be made is up front, so software developers don't get extra sales.
#2 The margins on $500 PC's are very low as the market is extremely competitive, certainly not close to the 30% margins Apple is used to.
#3 The quality of $500 PC's is very low as they have to use lower end hardware, either Apple would look like a rip-off at the price point or they'd have to use the same hardware which is extremely unreliable, many cheap computers break after about 2 years or so.
#4 Many buyers of cheap computers don't understand technology as they aren't willing to spend more, so they are more likely to get viruses/spyware, yes you are shutting out poor computer geeks, but they'd probably build their own PC anyway as the specs are better.
#5 With regards to point 1 and 4, they are less likely to upgrade their OS to the latest version, which means that the cool new features in the OS take longer to propagate, it's going to take 5 years for developers to release Vista only software using it's cool features, however in 2 years (maybe less for open-source/freeware) you will see Leopard only software.
#6 you can get a $500 Mac, just buy a second hand one on eBay, it'll probably last longer than a $500 new PC anyhow, as it's been made of better quality materials.*
* Though a high quality (business) PC laptop would probably last even longer
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