I had a mythv with a Hauppauge card and shaw for a few years. There is an add-on for mythtv that supposedly lets you control the Shaw box via firewire _AND_ it delivers the HD video over the same link but unfortunately the firewire was unstable. It would sometimes change channels then lock up or not send video etc. Could have been the firewire drivers, could have been the Shaw box, hard to say but I never ended up using it.
Otherwise the mythtv setup worked flawlessly on the regular non-digital cable channels and even though I no longer have Shaw I still use it to serve up content on my PS3.
Unless something has changed recently, you can not get digital or HD content from Shaw unless you buy their tuner. If the serial number of the tuner is not in their database, you can't use it. The CRTC should change this rule but the feds don't seem to be big on allowing competition in any telecommunication space.
Speaking of competition, I recently signed up for Netflix on my PS3 and after watching about 10 hours or so of programming, the service has been flawless. I'm very impressed.
Unfortunately the catalog is quite limited but if they happen to have some shows you are interested in then it's a good deal for $8/mo. It is the closest replacement for cable I've seen yet.
Regards,