Ummm... i do not agree that with US/Canadian degree one should take IELTS... Please read this..    http://www.cic.gc.ca/english/immigrate/skilled/language-evidence.asp    and if you are confident in each of the points described there.. you should get 16 points.

I know, even if you have phd degree from USA... when you write explanation in english.. and if you screw the english in that writeup... then they will doubt your english language ability despite your US degree...

Also, you need to show documentation to prove that you have worked in North America/UK/Australia in engish medium.... that again depens on how many years of work experience you have in these countries.

Here is the excerpt from that link:

You must:

1. Indicate what level of language proficiency you are claiming for each skill (speaking, listening, reading and writing).
2. Submit, with your application for immigration, a written explanation and supporting documentation as proof of the levels you have indicated.
   
   This material should include:
   • a written submission explaining your training in English or French;
   • an explanation of how you commonly use English or French;
   • official documentation of education in English or French; and
   • official documentation of work experience in English or French.