Our implementation will be to serve different mobile content under the same URL. We're trying to keep the content on the page as close as possible to the desktop site but remove weight from the page to make it much faster.
One of the most expensive elements of the page in terms of page load is our current internal linking navigation, which for the mobile version we're planning to drop and move onto it's own page effectively. This will have a great page load speed impact, but I'm concerned that Googlebot-Mobile not having access to our existing internal links might cause some sort of problem.
We're going to try it out on the basis that our desktop site will still be the primary source of ranking information, and the mobile versions of the pages will be indexed as alternative versions of that site rather than as a site on their own with their own internal linking structure. That seems like a reasonable interpretation of all the documentation I've read so far from Google etc.
Any counter opinions?