What's stopping me to create a new account, make 100+ short posts in different threads, and after that make the 100BTC bet that is auto-accepted? if I don't win I'll just drop the account. the IP is irrelevant (anyone with some technical knowledge  can circumvent this).

Anyone (including myself) who renigs on their bets will be labeled a scammer on the forums. Theymos will retain the IP addresses of everyone who has committed here and as you are marked a scammer for not paying, you will also be reported to the bitcoin police and tracked. For this reason, it is important that you do not bet more than you can afford to lose. Considering the high probability of fraud from newbie sockpuppets, only established 100+ post users will be allowed to participate.