https://www.paygonline.com/websc/rat...ateplan_en.jsp - rates are shown here, but not bad at all for occasional use (don't touch data though!). Travellers to Canada now have yet another option at their arsenal.
Nice to see now that both national GSM operators in the US roam in both Canada & Mexico (with T-Mobile very recently opening roaming up to a number of countries outside NA too, though at pretty high prices).
Active SIMs:
2007-05-14: T-Mobile post-paid (USA: 267)
2007-12: T-Mobile pre-paid (USA: 857)
2009-01-21: Mobal World (UK)
2010-06-08: TracFone (USA: 215)
2011-03-12: Tru (USA: 305)
2011-08-01: AT&T pre-paid (USA: 212)
2011-08-22: Spot Mobile (USA: 603)