Three things to keep in mind about Beijing Olympics:
1) The authorities have declared that reselling Olympic tickets above face value is illegal in China. (This will of course, be ignored by most or all of the Chinese reselling population.)
2) The authorities are not going to allow scalpers to stand outside the venues and hawk their wares. There has been no official announcement on this, but knowing the Chinese obsession with presenting a Perfect Face on these Games, they'll likely keep clean-sweeping all venue areas of anything viewed as pestilence.
3) There is so much demand from the Chinese themselves, that any last minute tickets from the general box office are likely to get snatched immediately, though they probably will not have access to any extra tickets at the international agents offices (cosports, sportworld, etc.).
I'm sure lots of trading and selling will go on, but it will occur in the hotels, cafes, company offices, etc. and may be more difficult for the average foreign visitor. Wish I had more encouraging news for you Blue, but if you don't want to use the ticket brokers (at this point, I'd be careful), you'll need to get very creative networking while still at home, and in Beijing. I think over the next month or so, people who got only a small allotment of tickets through official sources may decide it's not worth it to go, and will put their tickets up for sale on ebay, craigslist, etc. Maybe cheaper than brokers, but how to separate out the honest folks from the charlatans is a knotty problem. Also, Ceremonies tickets are registered specifically to the original purchaser and cannot be transferred at will, so these are an extra problem to validate and use, if you buy them on secondary market. Remember no actual tickets exist right now, only the "confirmation" that a ticket has been won in lottery or successfully open sale purchased.