Single currency is the last step of the process. The EU had to go through about 45 years of development, from the original 6 members to the current 27 members. Development included agricultural, educational, and production standards. There were standards for coal and steel and commerce and imports and trade. Some countries, like Turkey, have been trying to get into the EU for about 30 years and still haven't been able to get in because their standards are not on par with the rest of Europe.
I think before North America has a single currency, Mexico would have to achieve the same standards of education and economy as Canada and the US. I have a hard time believing this will happen anytime soon.
The currency is not the last step.. one single consitution for all countries within the EU is the last step (or so it seems at this point) we still do not have one. We got the Euro before that. France and us voted "Hell no" on the one constitution. But now our government is going to shuf that shit down our throats anyway, no matter the peoples opinion. I don't know about France but with Sarkozy it seems like they are going to do it aswell.
Also it is not necessery for Mexico to get the same standards as the US and Canada (I think the standards in the US are not that high anyway since there is so much poverty there aswell) I think it is not nessecery because Poland, Hungary, Greece etc are all members of the EU, altho they do not share the Euro yet.
Also the lire in Italy has always been very low one dollar was worth several honderds if not thousands of lire while one dollar also equalled one guilder (our currency before the Euro).
When we got our guilders changed for Euros they changed 1 euro for 2.2something guilders at the time while actually the guilder was worth much more but because this rate was so much more convieniant for the bigger countries like Germany we got screwed deeply.
I do believe there will be one currency for the whole northen American continent I also think they will do this a lot quicker then we did in Europe, they learned a lot of things what to do and what not to.
I also think what IAM said that the current paper and coin money will dissapear just look at internet banking since this is getting more used now my bank won't even send me my overviews on paper anymore I have to log on to a website to check my balance (or by phone). But I don't think I will live the day to see that.