Kevin, the answer to your question strictly speaking is: it depends.
There are several factors to consider in selecting either of the options you have described, and neither is "wrong" per se. It just depends what your goal and circumstances are.
That being said, scope and scalability of your site/application are the primary concerns.
I've done a fair amount of research on this topic for 6 mo. - 1 year now (for my work on CURE.org, where we've already implemented one of these options for cure.org/blog), and in fact revisited this discussion again this week for my personal site, joelglovier.com.
So when I say that "scalability" is a concern, you have to remember that if your site is going to have continually updating content, then you have to accomodate that in both places if you have your scenario of m.site.com setup. If you're site is database driven and/or uses a good CMS, that isn't necessarily too difficult to setup.
Nevertheless, depending on how often you are creating fresh content, landing pages, or campaigns on your site, it could grow cumbersome to have to create that content twice every time.
And if you're referring to a simple portfolio site for yourself or a client, then again repurposing content (without any type of database or CMS infrastructure) can get cumbersome quickly.
But on the other hand, when you have a site like Facebook, for example - where scalability of your site/content is critical to what you're doing as a whole - then a subdomain is really very appropriate. Partly because at the web application level you're most certainly going to have a CMS or dynamic environment that will fairly easily support your ongoing content publishing to both the mobile site and the desktop site environment (after initial development of course).
The separate mobile site really allows you to feed users only the markup and content that you want them to have versus serving everything and hiding much of it via the @media-queries method or a jQuery method.
Personally, for my own simple portfolio site I've more or less settled on not having a separate mobile site because I'm not building anything that is really application level. So I'm going with CSS3 media queries, and jQuery fallback for IE users.
Here's some of the recent resources I've been thumbing through: