WirelessRadio.net - FCC Part 90 VHF UHF Radio Narrowbanding Info, Resources, & Compliance Updates has been a great resource for me in learning about this issue.
All I can find there is that two watt gear is presently exempt from 6.25 khz certification for efficiency mode capability.
A document on that site from the FCC Engineering and Technology department stated that any two watt radios used in conjunction with repeaters would have to meet the narrowband requirements. That is all it said.
An Industry Canada Narrowband Mandate went into effect January 1, 2004. I found this on one of Motorola's FAQ sheets for narrowbanding:
How is Narrowbanding being managed in Canada? If I operate an LMR system in VHF or UHF in Canada do I follow the FCC requirements?
Licensees operating in Canada are not subject to the FCC rules, but instead are regulated by Industry Canada, which also set Narrowbanding requirements for all VHF and UHF land mobile radio equipment. While the intent is similar, the rules and deadlines vary. Similar to the U.S., since February 1997, all radios certified after that date were required to include 12.5 kHz efficiency. Industry Canada licensee dates are generally more aggressive than the FCC dates, requiring that as of February 1997, all new license applications must be in 12.5 kHz efficiency in the urban spectrum congested areas, and that as of January 1, 2004, all existing 25 kHz efficiency systems are considered “non-standard” in those spectrum congested areas. Industry Canada however, excluded the less populated areas from required Narrowbanding, and has not been consistent in enforcing these rules in the urban areas. While it also defined rules for meeting 6.25 kHz efficiency, it has placed a moratorium on those rules, which it will likely not lift before the FCC makes any future decisions. Licensees should contact their Regional Industry Canada office for Narrowbanding requirements in their area. http://www.actcom.net/Narrowbanding/RO-99-2227.pdf for more.
I don't know exactly what is going on in Canada currently compared to when the FAQ was written, but I am seeing Ebay auctions for WB gear coming out of Canada now. That suggests to me that the Canadian market for used WB only gear is probably going to shrink pretty quickly.
A side view suggests that both the US and Canada aren't setting any deadline for 6.25 yet. (Whew!)
Yes, the hams will have a vast supply of WB gear to pick from, but as I said before, it is likely there will be a lot more equipment available than the hams can absorb.
I really think a lot of this equipment is going to end up alongside old PC's at the salvage/recycling depots.