The Nintendo 3DS update which will allow players to StreetPass with one another via an internet relay will launch in "a few weeks," according to Nintendo of America's Bill Trinen in a Nintendo Direct posted today.
The specifics of how the relay will operate were unclear when the feature was first announced by Nintendo president Satoru Iwata during an E3 investor's presentation. Today's video makes it a bit clearer: Anyone who stops by an authorized NintendoZone location will be able to download StreetPass data from the last person to visit that location. After accessing that data (which can be utilized in any StreetPass-capable game or the Mii Plaza lobby), your own StreetPass data is uploaded, where it will be shared with the next person to stop by that particular NintendoZone.
You can check out Nintendo's site for a full list of NintendoZone-equipped locations; a list which includes no small number of Starbucks, McDonalds and AT&T retail stores.