[GadgetKen]

As a US citizen with a Fido card, I agree that ebay for a prepaid sim is probably the way to go. A few thoughts:
1. On the T-Mobile call forward for your BlackBerry, you may want to exchange an email with T-Mo just to confirm in writing you won't get charged for call forwarding to Canada while the phone is switched on.
2. If you go the Fido sim card route, remember you may need to change your WAP/GPRS/MMS settings temporarily on your HTC if you want to use this as a data backup for your Blackberry (see the faq section of Fido on http://www.howardforums.com for details on this). Fido prepaid has roaming in the US as of a week ago (data rate same as Canada, but voice a pricey C$2.50/min so wouldn't recommend using voice other than maybe a one minute test call-out or call-in) so you can test your phone in the US. You can also setup your voicemail box from the US by calling a Fido VM access number for the area your sim chip is set up for.
3. Have you considered an iDen Blackberry for Canadian data? You would need a cheap used Nextel blackberry handset off ebay. Roaming is possible on a Boost/Nextel prepaid sim chip on the Canadian Telus network (mostly CDMA but some iDen Mike coverage). Cost is only 35 cents a day for unlimited web service. PTT service also available but incoming/outgoing calls not allowed in Canada, so you would still need the HTC on a Fido chip for voice.
4. Free or fee wifi available in many major cities in Canada. Bring a subnotebook or wifi device if your Blackberry or HTC doesn't have wifi capabilities? Wifi is generally cheaper for data and sometimes faster than GPRS.
5. If going Fido prepaid, this can be refilled by voucher from your handset or by calling a 1-800 number (both free calls in the US and Canada). You can't use credit card refill unless you have a credit card drawn on a Canadian bank (unless buying indirectly from a Fido reseller).