The streetpass ads show it working between pedesrians and cyclists, though I've never had a hit when I'm cycling. Although that probably doesn't mean anything as I've never even got a hit sitting at the lights.
i was hoping that would be the case cos i cycled down oxford street, past hmv & game, etc, and not a sniff!
It depends how fast you are going, we put a system in debug mode at work and had a nosey at how it worked and it listens constantly but only broadcasts very sporadically, sometimes it goes 20 seconds without broadcasting. I guess the theory is you only need one of the two to broadcast for the sync to take place, but in practice you can be right next to someone for 20 seconds and not get a hit if your unlucky with the timings.
If you go out of range before the sync is completed it's null and void, and a system can only sync with one other system at a time. The firmware seems to limit it to 10 syncs per sleep cycle, not too sure why - it doesn't seem to process the data until it's pulled out of sleep so I guess it's to stop it becoming too unresponsive.
The broadcast range of the system also seems to be reduced from normal WiFi mode, I guess they put the chip on a low-power mode or something, the limit dropped from like 20 feet to a few feet once it was in a hardcase and a coat pocket.
It's a great concept, but it seems the concern for battery life has somewhat crippled it for now.
It seems you can only have 5 games set up for street-pass at a time too, not that it's too much of an issue at the moment but once I get a larger collection it will kind of suck to have to disable the earlier titles.