"Feature" Phones, "Smart" Phones and "old skool" Content:
There are two types of mobile content: traditional mobile content (e.g. WML, WAP based content); and smart-phone content.
Google tends to refer to phones that handle the old-skool traditional style of content as "feature phones". It has a specific mobile crawler to crawl that old-style content (you can find a full list of Google crawlers here).
Other more modern phones are unsurprisingly referred to as "smart phones" - iPhones, Samsungs, etc and so on.
I argue that you only need to actively engage with the Google Mobile crawler when you are serving "old skool" "feature-phone" type content i.e. WML, WAP-based content. For all other content, you can simply leave the Google crawlers to spider the content as normal. You may want to inform the new Google smartphone crawler where your highly-optimised smartphone specific content is, but you don't have to.
You do however have to tell Google's mobile crawler where to find any "old skool" WML/WAP style content.
There's no requirement, no need, no benefit from serving any mobile content from a sub-domain. You don't need to - unless technically your digital platform requires it, or its more simple (and hence less costly), or some other reason. But from a search and crawler point-of-view, you do not need to use a sub-domain.
You could serve traditional mobile content from the same URLs as you serve desktop, smart-phone and all other content - you just need to examine the requesting user-agent and ensure that you serve the right mobile content on the same URLs to the appropriate mobile phones and mobile crawlers.
In other words, all your URLs could look exactly the same, but serve different content to different phones and different crawlers. This might not work for all content, and you do have to be careful that you can't be perceived as cloaking, but your URLs could be exactly the same.
If I Do Need a Sub-Domain?
If you do decide to serve traditional mobile content and you decide to serve it on URLs distinct from 'www' (e.g. http://m.example.com/), then Google appears to suggest that you serve the 'rel-canonical' tag on the traditional mobile content referring back to the equivalent desktop URLs on 'www'.
See the "John Mueller - @Paul If you have "smartphone" content (which we see as normal web-content" discussion at https://profiles.google.com/softplus/posts/38v3DayB75g with John Mueller of Google for a useful set of comments and opinions.
But First, Market Research: What Do Your Current and Potential Visitors Actually Use?
Given that serving old-style traditional WML/WAP based content might be costly, the first thing to do is to know your visitors - does their use of feature phones actually justify the expense of serving feature-phone specific content? This of course is an argument that applies to any specific content - iPhone, TV, and so on.
So the first step has to be market research: what do your visitors - and in particular your potential visitors actually use?
Regarding smart-phones, there are a number of excellent articles discussing how to create smart-phone specific content (for example using the 'media' operator in cascading style sheets):
There's also a set of forensic mobile specific articles on SEO Moz (where else?):