In this July 12th, 2010 letter to Edward Markey and Joe barton, Apple details how and why it collects the user data to update its location database. From reading this it seems that Apple only grabs this anonymized data under two conditions:
1. You are requesting your location. At this point Apple grabs any updated information.
2. You are using another app that uses location services, at which point Apple will update its data.
Of note: you can stop all this from occurring by turning of location services.
[h/t reader Jose Marques]