Anyone using connecting/disconnection from WiFi as a presence sensor

Some people are doing it. See the presence FAQ:

Or check the quick browse lists in the community – created wiki in the project report section on the list for presence:

http://thingsthataresmart.wiki/index.php?title=How_to_Quick_Browse_the_Community-Created_SmartApps_Forum_Section#Quick_Browse_Links_for_Project_Reports.2FQuestions