We have a mobile version of our site under a subfolder, something like example.com/m/. We are using some mobile meta tag like HandheldFriendly, MobileOptimized, apple-mobile-web-app-capable.. and that's all. We have the same title and description for every mobile page.
Now we are planning to generate a unique title and description for every page, create a mobile sitemap and we were thinking about using the canonical tag pointing to the desktop version and the alternate tag pointing to the mobile version of every page. Is that approach still the correct one for SEO?
I'd say start with Google's own Help articles on the topic: Making Websites Mobile Friendly [googlewebmastercentral.blogspot.com]. Lots of good information there, including mobile googlebot, user agent detection, and URL structure.
For Googlebot and Googlebot-Mobile, it does not matter what the URL structure is as long as it returns exactly what a user sees too. For example, if you redirect mobile users from www.example.com to m.example.com, that will be recognized by Googlebot-Mobile and both websites will be crawled and added to the correct index. In this case, use a 301 redirect for both users and Googlebot-Mobile.
If you serve all types of content from www.example.com, i.e. serving desktop-optimized content or mobile-optimized content from the same URL depending on the User-agent, this will also lead to correct crawling by Googlebot and Googlebot-Mobile. This is not considered cloaking by Google.
Here's another resource from our own Accessibility and Usability forum. If you can, use a bidirectional redirect so that a mobile user who lands on the desktop site gets sent to the right site, AND vice versa.