Here's how it works:
1. You submit an idea about how you'd use the iRobot Create to do something cool to the iRobot group forums.
2. We choose the top 15 entries, and give them iRobot Creates.
3. People who win iRobot Creates build stuff using them, and submit them to the main contest.
The only caveat is that you need to submit an entry to the contest if you win one of the Creates, so the ideas should be something you feel confident you'll be able to do. You can submit more than one idea, but we'll only be sending one Create per person, and will look unfavorably on idea spam. Please think through what you're going to do.
If you don't enter the main contest, you have to give the Create back. If you do submit an entry into the contest, you do not. You do not have to win the contest to keep your Create, you just need to submit an entry.
All ideas need to be submitted to the group by June 30th. The prize for the full contest is $5,000 cash.
Full rules are available in far too much legalese on the the original contest page.
Update: check out PopSci's first lesson of their iRobot iNstitute.