The highest wagerer is currently at 70BTC, that's higher then I'd have expected. I was going to put in 0.1BTC and get a bot running to gamble with 0.001BTC and see how high I could get (with 0.01 odds) but that seems a bit dishonest - though theres still an equal chance to gaining and losing the same amount. I'm not sure if anyone's done this though (there's probably a slim chance you'll get profit from it or your betting will at least last a bit longer on the site).
And only halfway through the month. The race for second has been hotting up a bit this week as well. I still wouldn't be surprised to see a really big high roller come in and take it though.