@Scott - I think the theme/plugin is a better way to go, since it allows for things like switching to full site, and it means mobile themes are swappable; if someone develops a better mobile theme, you just replace it.
@Another one - sorry, I just meant to say if you want it to look different, change it yourself. It's all pretty stable, though one of our users reported that after visiting the site on her phone, she was directed to the mobile theme on her computer as well... I can't reproduce that, and it looks like the code won't do it, but there may be a bug I can't see.