If Public Mobile were to introduce SIM cards at 7-11 locations across Canada, it would vastly improve its reach. Plus, it would be convienient for newbies (like myself!) to purchase a SIM and a top up code at the same brick-and-mortar location.
This has been discussed before... Given that PM would directly compete with 7-11's own prepaid phone service (SpeakOut Wireless) - and at a better price/value no less - there is not a lot of incentive for 7-11 to carry PM SIM cards and/or vouchers.