I completely agree with you Swarf. You probably noticed that I was a victim to this as well, having my score drop from +90 to +67 in less than 5 minutes. It's a little sad that we aren't being judged on our work in this competition, we're being judged by who can beg or bribe the most friends to vote for their work and downgrade other people's entries.
I refuse to ask my friends to vote down other entries, since I'm not going to employ the same tactics as others who think this is a legitimate way of promoting their own entries, by degrading others. I've submitted links on a few machinima websites, which I encourage others to do the same. Then people can come here and vote on the entries they like best, not because they feel they have an obligation to that person. Really, the biggest problem here is the negative vote button. Without that, it's a tally of votes, and not a popularity contest.
Consider this like a running race. At the moment the winner isn't who can run the fastest to the finish, it's the person who can trip and take out as many people along the way as possible.
--Edit--
Here's an example of what some people are doing to get votes. This has been spammed on a WoW irc chatroom by Ghwrin.
[ Hourly Message ] Please upvote my contest entry at
http://www.gameriot.com/contests/blackbird/kqhKfl8MPK/ and downvote
the top 2 or 3 videos in the Most Popular tab in the right hand column.
You don't need an account to vote!
Take note that he's actively asking people to downvote other people's works. I'll leave this to you guys to decide whether this is acceptable practice or not.