I'm an American Citizen who lives in Washington DC. I work as a consultant. I have a current work permit that expires in Feb. 2008. My current client in Canada (in Montreal) involves probably 1 trip a month to Canada for work for just a few days.

I've since been approached by 2 other Canadian organizations (one in Vancouver, the other in Toronto). Each would like to to do some limited work for them (probably a week's worth for each organization).

This work would likely happen in the Fall of 2007 sometime.

Does my current work permit cover this? Or do I need to get a seperate work permit for each client?

You will need separate work permits.