As a first-time entrant, this is exactly the dynamic that I am struggling against.
I’ve seen some designs (that I think look a little… ah… “not ready for prime time” that are nonetheless scoring high because they have hundreds of followers.
So I actually think like you, and have suggested to people who have PMed me to only vote what they feel (there’s no way for me to know what an individual truly voted) and share the upvotes to any of the other great designs that are also competing.
I see nothing wrong with posting about it asking people to vote for you IF they feel your design is worth voting for. You aren’t asking your friends to vote for you regardless of whether or not you like your design, which indeed is unethical and wrong. So I think it’s fine.
And I remember the one about the poster asking people to vote for a Pepsi sponsored thing that would give smartboards to their spouse who taught autistic children, I think.
I also remember a fair amount of begging that was allowed (for an overheating bunny, for job postings, and other things) which is in the same vein as “please vote for my threadless design if you think it’s worth voting for.”
For what it’s worth, I hope this gets allowed somewhere, even though I probably won’t even click the link or vote for you (sorry, I just don’t care that much).
Heh. That is a perfectly reasonable response as far as I’m concerned.
In the early days of the internet, I entered a JPEG of one of my tattoos in a contest (back when a JPEG loaded line by line). I asked a bunch of friends to vote for me. One of my friends was a computer geek and an early spammer. He posted links to my tattoo in various newsgroups, etc.
So I won. I still have the trophy. I got around 5,000 votes to the second place’s 23 votes. I was embarrassed. I called the sponsoring shop to confess and apologize, but they were just thrilled with the exposure and reassured me that I was the winner regardless. Good times. 
B. Serum, could you send me the link too? Although I’m guessing it’s this shirt by user “bserum”, right? http://www.threadless.com/captainamerica/project-rebirth-2/
That is it, indeed.
Actually, at long last, I was cleared to post in the Marketplace (with one day to spare).
Link to thread. (with picture preview!)