Making a crypto currency GPU immune is a stupid and naive goal.

1) It still won't be "fair".  Sure if you can only use CPU then the traditional mining farm because kaput.  It still doesn't make average user an "equal share".  What about IT department managers who may have access to thousands of CPU?  They dwarf the returns than an "average" user can ever make.  You simply substitute one king of the hill for another.

2) It makes the currency very very very vulnerable to botnets.  The largest botnet (Storm Trojan) has roughly 230,000 computers under its control.  It could instantaneously fork/control/double spend any crypto currency.   There are much fewer computers with high end GPU systems, they are more detectable when compromised, and on average tend to be owned by more computer savy users making controlling an equally powerful GPU botnet a more difficult task.

3) If GPU were dead then FPGA would simply rein supreme.  CPU are still very inefficient because they are a jack of all trades.  That versatility means they don't excel at anything.  If bitcoin or some other crypto currency was GPU immune large professional miners would simply use FPGA and drive price down below electrical cost of CPU based nodes.  The bad news is it would make the network even smaller and even more vulnerable to a botnet (who's owner doesn't really care about electrical costs).

4) Technology is always changing.  GPU are becoming more and more "general purpose".  It is entirely possible that code which runs inefficiently on today's GPU would run much more efficiently on next generation GPU.  So what are we going to scrap the block chain and start over everytime their is an architectural change.

5) CPU will become much more GPU "like" in the future.  The idea of using multiple cores with redundant fully independent implementation is highly inefficient (current Phenom and Core i designs).  To continue to maintain Moore's law expect more designs like AMD APU which blend CPU and GPU elements.  Another example is the PS3 Cell processor with a single general purpose cores and 8 "simple" number crunching cores.  As time goes on these hybrid designs will become the rule rather than the exception.  Would be very silly is any crypto currency was less efficient on future CPU designs than current ones out of some naive goal of making it "GPU proof".