Is it possible to show a different content in a post when it's on mobile or not?
After a forum request, I wrote Mobile Comments Signature plugin as a project. It recognizes a comment's user agent as mobile or not and appends user-defined text if it's a comment made from a mobile.
You could take a look at it for inspiration if you wanted. It's pretty well commented and it's got a mobile user-agent detection script in it (which I found on the web, credit given within the plugin).
Although I've found (I think) that checking the user agent of a post is not as easy as checking the user agent for a comment. WordPress stores a comment's user agent in the database to be accessed at any time, but doesn't seem to do the same for posts.