I don't really see this having a big impact on most people. Do they want to spend $2300 on a Mac Pro which is currently a better deal than a comparable Precision, or $1300 on a Dimension which is plenty of machine for their needs?
Apple is giving a good deal on those, there's no doubt. The only way I could come close in cost/performance building it myself appears to be if I went with the slowest chip Apple offers since they aren't too astronomical online. However Apple is buying in "bulk" and prices will gradually drop (new chips are always inflated) and Dell is likely to fight back.