There's no direct SEO benefit per se, just the mitigation of duplicate content that can be introduced when an m.dot sub-domain (for the same content) is incorrectly implemented (i.e. not using rel=canonical/rel=alternate and other indexation directives). There's also the 'crawl once, show everywhere' benefit if you have a single front-end representation of the code. Introducing an m.dot version of your site requires Google et.al. to consider whether to use their separate mobile crawlers to crawl the content that has been created for the mobile experience.

In most cases you wouldn't want your mobile domain indexed by Google et al. so simply block all crawlers in robots.txt.

Seperate subdomains seems like a monster pain to me.

You can actually use media queries to link separate style sheets: http://www.w3.org/TR/css3-mediaqueries/#media0