We just added Google's new "+1" button on our homepage, and so we're testing it on multiple browsers... and noticed that it doesn't show up on mobile devices.
We haven't found much information on this yet, other than a single thread on Google Webmaster Central Forums. This thread had a Google Employee stating: "At the moment, we don't support mobile devices with the +1 button. It's my understanding that this is something that's being worked on, but at the moment they're not yet supported."
Here are a couple of screenshots highlighting the issue:
Homepage in Firefox 4 (standard):
Homepage in Firefox 4, with the User-Agent set to iPhone: