Starting April 1st, 2008, you have to post every day. Even holidays. Even when you're sick. Even if your house blows up and you get abducted by aliens. That's the "iron man" part. If you fail to post every day, you get disqualified. The last guy standing wins.
Each of your IMBC posts has to be worth at least 300 CV points. Points are given for a few different things, but mostly word count. You get 1 point per word in the post (we use MS Word as our official word counter). You also get 50 points if you post some kind of media you made yourself (an image or photo, a video, a sound file, whatever) and you can snag an extra 25 points if you devote at least 60 words in your post to the IMBC "Topic of the Day." Oh, and be careful about quoting text you didn't make up yourself -- only the first 75 words of that counts towards your score.
Make sure your posts make sense. Don't let them get too repetitive. Don't plagiarize. Post in English. Make sure your site stays up. Keep your old posts (if you don't keep them on your site, at least keep local copies that you can send to the organizers if they ask for them).
And some advice: Don't cut it close. A post that's exactly 300 CV points is risky. Give yourself a little wiggle room — writing an extra ten or twenty words won't kill you.
Finally, know that this summary is in no way complete. It's probably enough for you to get by on, but wouldn't it suck if you got disqualified because of one little rule that wasn't mentioned here?