It's been out a while but I haven't seen it mentioned here: Telcel Mexico have a new prepaid option: "Amigo Óptimo Plus Sin Frontera".
http://telcel.com/portal/personas/am...html?mid=1114#
Free incoming calls in Mexico, USA, and Canada. Same price to make calls whether you're in Mexico, USA or Canada, calling any of those countries (from 0.98 to 1.98 MXN / US 6 - 12c per minute, depending on how much you recharge each month).
Outbound texts are 0.98 MXN / 6c, data is 1.98 MXN / 12c per MB, again from any of the three countries.
Minimum reload to keep the account alive is 100 pesos every 60 days (so US $3/month). One downside: Telcel only roams on T-mobile in the US now, rather than AT&T.
I use this sim in combination with a Localphone account where I have incoming numbers in Canada ($3/mo), US and UK ($1/mo each). I answer that on (or call from) VOIP when indoors, and it forwards to the Mexican mobile with this tariff when I'm out of wi-fi range, at a forwarding cost of 6c/min.
Works well for my low voice-only usage - I can be reached on the same phone from local numbers in 4 countries, I pay 0-6c/min in, 12c/min out to US, Canada, Mexico, when not on VOIP, and total cost is around $8/mo.
BTW, Movistar (the other main Mexico carrier) do have a "call from the US at the same rate" promotion at the moment, but they charge (at the same rate) for incoming while roaming, and don't cover Canada.
(Edit: there's a typo in Telcel in the subject, but I don't see any way to edit that).