What about for people who don't have a specific idea (or none small enough to reasonably start on in 54 hours) but want to hang out and help on someone else's idea?
I just booked a ticket after seeing the article here, and I'm pretty excited. Even if my idea doesn't win any votes, It'll be a fun challenge for the weekend at least and just possibly a big opportunity.
Also, reading around I found a discount code for the event here: http://www.cabume.co.uk/the-cluster/inaugural-cambridge-star...
We want to get people along to try out their ideas, and also who just want to collaborate on great projects! Great startups are born of great teams of people, who are much more important than any one idea.
Today we have also announced special prizes for Wireless and Healthcare projects, as well as our overall prize for best project.
To find out more, look here - http://cambridge.startupweekend.org
If you're a developer or designer I'd definitely recommend hanging around, they always seem to be in short supply. 54 hours isn't a lot of time to start a company so teams will take as much help as they can get.
As for an idea that's 'too big', if you're really in love with it and want to pitch it I'd recommend segmenting it. Try to find a feature or a subset of features you could build in a weekend with a team. SW is what building a startup would be in a perfect world, where you have everyone and everything you need right at your finger tips. But it only last 54-hours, so capitalize on it while you can.
Also, if you need help with your pitch just tweet before the event using the hashtag and ask for help. That's what I did and I got people (namely @JoshSamBob) jumping in to help even before it started!