Say you and 5 of your buddies get together and decide you're going to get to 10,000 reputation in 6 months. How difficult would that be? Here is my approximate math - 10 points each for an upvote plus 15 for the accepted answer(accepting the answer too often might start to look suspicious but bear with me it's an approximation) - you would need to ask/answer approximately 154 "fake" questions - with some variation in your voting not taken into account. That seems do-able in six months - right?
Now I originally proposed the idea of a vote history - so you could see who is working together to game the system - Now I understand the drawback of this would be that it would destroy vote anonymity(which I don't really value that much for myself - so I didn't consider how important it was to everybody).
So all that aside - I want to propose another idea about how to combat "mutual upvoters". You could have a vote density measurement within tags on each person’s profile. So assuming these 6 guys all upvoted each other and nobody else upvoted them and they did it within just one tag (or a variety of tags - it doesn't matter in this case since they would be consistent in their upvoting) - each of them would have an upvote density of 20%. If it was just 5 of them it would be 25% - three of them 50% and 2 of them 100%.(or if you did this based on points - the percentages would be slightly different - I'll let you do the math) So your goal would be to have a low density of upvotes. Most people currently on SO rarely answer questions from the same guy over and over and over again - so the average person would have a low upvote density(probably well below 5%). You could probably do the same with a downvoting density within each tag - in this, you would want to have a high density of downvotes (that way if just one cranky guy is repeatedly downvoting someone - you will be able to see that).