I met the guys from Winterborne at the Bike show last weekend. Their stuff was actually REALLY nice. Their show bikes had a lot of nice detail in the lugs and the paint jobs were beautiful.
The True North guys are amazing too. I am not sure if they STILL do, but they were building Cervelo's TEAM bikes for them.
RUBY CYCLES is another custom frame builder, and they are located in Toronto. They are very small, but they do a fantastic job too. We are actually going to be carrying Ruby Track bikes on our website this summer (they are in the shop now). All Steel!
Glacier Bikes is another Canadian company. These guys are a fairly new company, and they are doing only carbon bikes. I believe these guys import their frames, but they had a lot of design input with the manufacturer. They are only planning on making 100-200 bikes per year...So they may be tough to get.
For custom modifications to steel bikes (like adding track dropouts etc). Mike Barry is one of the best around. He owns Bike Specialties in Toronto. He's done some work for me, and it was second to none.
Well thats all I know. Hope that helped.