I am finding that the language people use when speaking/writing about mobile site versions has a lot of confusion, which is in turn confusing me! I am considering your plugin but would like to be clear.
When I read about mobile themes it sometimes turns out they are a plugin, sometimes an actual theme. In your description and notes you discuss the code that determines which theme is served for a mobile device. When you say "theme" is this an actual theme placed in either a subdomain, sub-directory or separate url, or are you referring to a plugin?
If you are referring to actual themes, have you found it makes a difference if they are either subdomains or sub-directories?