Hmm, popular mobile browsers cannot see the +1 button. I tried on an iPhone 4 and iPad with latest firmware. Same with an Android Nexus One with 2.3.4 (using the built in browser).
It DOES however work with Android with the mobile Firefox 4 and 5 beta browsers, and Palm Pre 2 WebOS (latest firmware). With the Pre 2, I had a lot of dialogs pop up to ask me to trust Google’s certificates, however.
In their FAQ Google says it “may” be supported:
https://www.google.com/support/webmasters/bin/answer.py?answer=1140194
Q. Will users on mobile browsers see the +1 button?
A. We are not including the +1 button on mobile search results at this time. However, users may be able to see any +1 buttons you have added to your web pages.
My guess is, there is an untrusted cert issue through one of the Google API calls, which just fails silently (which WebOS handles and Firefox doesn’t care about).