Pretty much anywhere online that votes can be automated, they'll be exploited. It's unfortunate.
I'm co-running a jam in July with under a couple dozen people. Personally I find the comparison of what you made between other people you know much more interesting than the dozens, hundreds of people online.
Ironically as game development becomes easier and easier to get into.. well, anyone can get into it and lower quality games + lower quality people (vote manipulation) comes into effect.
I think LD would work great strictly for small communities -- groups of friends, specific cities etc. Kind of how Global Game Jam works.
While I'm on my rant.. I found the theme voting uninspired. I wish there were photos or songs as references, something like the fake album challenge
http://www.synthtopia.com/content/2009/03/24/name-your-band-create-your-album-cover-with-wikipedia-flickr/.
There was a recent low-res jam and I liked that -- constraints breed creativity.