301 the .ca to the .com, IMO. G will pass the PR, backlinks, etc from the .ca to the .com site. You don't want two sites with the same content for possibility of duplicate content penalty. If you have the site in G Webmaster Console you can also tell G of the move via the console, which will help you to tell G of the move.

I suppose I should clarify further. As part of the "website upgrades" I will be doing for him we will be changing the page structure as well (going from static html page to a CMS). So there must also be 301's for these page level path changes.

So should I then 301 from specific pages like 'mydomain.ca/somepage.htm' to 'mydomain.com/somenewpage.aspx' or should I just use a domain level 301 for all pages to the new domain and then resolve page level redirects on the new domain? (Can double 301s can cause some penalty?)

- Ben

301 from specific pages like 'mydomain.ca/somepage.htm' to 'mydomain.com/somenewpage.aspx

Yes, that's the best way to do it. 301 redirects, one step.

Do everything you know how to make sure the redirection from an old URL to the new one happens in one step. If that means creating a zillion individual redirects, do what it takes. Chains of redirects don't exactly cause a penalty, but they're not good for your SEO and should be avoided whenever possible.

Make sure that the redirects are set up and ready to deploy within seconds of when the new site goes live.

Good redirects deployed promptly are probably the single biggest factor that will reduce ranking problems during the transition. Follow up by making sure that all links under your control are updated to the new URLs, and also do what you can to request that other sites update their links as well. Give priority to links that send the most traffic, have high Page Rank, or that you think might have strong "trust rank".

Do some research NOW to identify as many of the old site's inbound links as you can, because some of the free tools for that have been disappearing.

As buck said, redirect page-to-page. I believe Google even recommends this in their FAQ for redirects.

you should probably follow the links i posted in this thread last week:
How to change website from .COM to .ORG while preserving Pagerank and inbound links [webmasterworld.com]
although in your case you are moving from a ccTLD to a gTLD, there is much that is relevant to your situation in those references.