Max, I never got a good answer on this, but when we put up a mobile version of our main site about 2 years ago, Google indexed some of the pages in their main index which caused lots of problems - it cut our page rank in half and it still hasn't come back.
We had the correct headers, the m.example.com subdomain and a mobile sitemap, but Google mangled it anyway and I still don't know why. The canonical tag wasn't around back then, but I would definitely use that now, and it might protect you. We had to noindex to get them out of the main serps.
At this point, I'm not sure a mobile site is worth it, especially in a world of apps.
Yeah the canonical tag makes a lot of sense for mobile pages. We also submitted a mobile sitemap. It was very confusing to sift through all the varying opinions on mobile site design. Hopefully we got it right. Great point on the apps. We are looking to build an iPhone and android app next.