This was reported over E3 but didn't seem to get a lot of attention. It seems that the August firmware update for 3DS will change how Streetpass works here in Europe and the US. Nintendo said that it worked well in Japan but that numbers were really low everywhere else.
The new system will collect and send Streetpass data when near public wifi like Starbucks and McDonalds.
I've only ever managed to use Street Pass with friends and occasionally when walking near game retailers.
Hopefully this will make the feature much more worthwhile.
Quote from Nintendo:
In its E3 Analyst Briefing, Nintendo revealed its plan to release more software that takes advantage of StreetPass functionality, and in a bid to increase its popularity in the West, it's planning on using public Wi-Fi access points – such as those in Starbucks and McDonald's – to provide 28,000 Wi-Fi access points in the US and 24,000 in Europe.
"This will involve taking a Nintendo 3DS system in Sleep Mode to an access point which will then connect automatically and send StreetPass data to a server. At the same time, the Nintendo 3DS system also receives the StreetPass data of another user from the server. In this setting, data is not exchanged directly, but rather through a StreetPass relay station."
"Previously the StreetPass feature required multiple Nintendo 3DS systems in Sleep Mode to be in the same location at the same time, but in this framework, you can exchange data with others by visiting the same location even at a different time, so we can certainly expect the use rate of the StreetPass feature to grow significantly."