Fido is a Canadian GSM provider, using the 1900 MHz band. Fido is now owned by Rogers and they offer access to the Rogers network as an "Expanded Network" option on some plans. Note that in some areas the Rogers network uses 850 MHz, and this band appears to be disabled or otherwise non-functional on current Neo1973 handsets. 
See http://www.fido.ca/portal/en/packages/plans.shtml for information on Fido rate plans.
mmontour 20:58, 2 August 2007 (CEST) - I transferred the SIM from a Fido Prepaid phone into my newly-arrived Neo1973 and was able to send and receive calls. I have not yet tested any other functionality.
jeffnorman 22:39, 4 November 2007 (EST) - I can confirm a working Fido monthly package SIM, used to place and receive calls in a Neo1973 GTA01Bv4. n.b. this is a very old SIM (8+ years).
IanS 5 August 2008 - FreeRunner as shipped with 2007.2 software. New 2000 series Fido SIM and separately purchased prepaid airtime. Placed calls, received SMS. SMS and missed call notifications working.
FreeRunner Update @ 11-February-2010 User: Dale Rossetti
I was somewhat disappointed to learn that Rogers and Fido are the only providers which will support a GSM 850/1800/1900 phone. I tried someones Fido SIM card (from her iPhone) in a FreeRunner, and was unable to get data. I did not try voice at the time.