I know that tickets go on advance sale across the world in countries participating in the Olympics, each with their own quota. But this is for advance bookings. If one doesn't have firm plans and happens to be in London / England during the Olympic Games, is there any way to buy tickets at the venue itself (perhaps only for specific events) or only a week or two in advance?
There will be box offices open outside events. Some events are selling slower than others. This is especially true of events outside London such as the football in Glasgow. You can see online at www.tickets.london2012.com/ what events are available. Buying online near the time will require you to collect the ticket at the box office outside the even. Simply take your photo ID and the card you used for payment or the booking confirmation email and you can receive the purchased tickets.
Additionally, some people may cancel there plans to attend the games. There is an official route on the www.tickets.london2012.com website for reselling tickets at the original price only. So even some of the big events might have a few tickets on the day due to cancellations.
Personally, I've already got my tickets. They arrived in the post yesterday.
From 11am UK time on Wednesday May 28, the final batch of tickets goes on sale.
It also seems likely that some events will not be fully sold out - especially events not as high-profile in Europe and the UK - archery, handball and more.
Finally if you're really desperate to see anything, events like the marathon are open and free to viewers along the majority of the route :)
I suspect there's also a black market of sorts for tickets, but I wouldn't recommend going down that route.