I posted this two days ago in another thread:
OK I know you need a separate ticket to visit the Olympic Park, but how much is it, how do you get one, can you just turn up and pay?
Their website is useless, I can't find any info and it doesn't even say anything about tickets:
http://www.london2012.com/spectators/venues/olympic-park/EDIT
I've found it buried away in the tickets website FAQ
"Can I get a ticket to the visit the Park without watching any events?
There are currently no tickets available for entry only to the Olympic park itself across all dates."