While the MacPro may currently be cheaper (I wonder if that will last) it really isn't relevant in a sense. Sure apple fans will throw their money at it, but it's vastly more computer than 99% of the people buying need. Most people don't need to spend more than $1000 on their computer, nevermind $2300+ for the Mac Pro. This is the same reason most people aren't buying Precision workstations, if they want a dell desktop they get the Dimension.
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.