Can unlocked cell phones be used by any carrier?

I live in Canada and as you guys know we don’t have much selection when it comes to cell phones (among many other things )
The phones I am interested and want to buy are as follow
– droid x, htc incredible, htc evo 4g, and many other great android base phones on the market!!
none of the above listed above is yet available in Canada and not a single word on when it will be coming to any of our carriers
so my question is if I buy unlocked version of these phones from other country will they work in Canada??

and lastly I know HTC desire is coming to telus very soon!!!
what do you think of this phone??
can this phone be compared with samsung galaxy S series??