A Cre8asite Forums member asks how a site is differentiated by Google as a mobile site or as a regular website that may not be included in Google's mobile web search. What are the criteria for being categorized as a "mobile" site?
The Google Webmaster Help Center has a guide of questions and answers on this very topic. Additionally, other feedback is given from a webmaster who has been designing for the mobile web for a year.
He says that you need to avoid tables in your design, have concise titles, have a small page size, and include good content.
As far as understanding whether your site is going to be accessed, he says that Google crawls your site as a Nokia 6820 which accepts XHTML.
And finally, it helps to create a mobile sitemap.
Forum discussion continues at Cre8asite Forums.