Jeremy Herve on "[Plugin: Jetpack by WordPress.com] mobile theme, "read the rest of the story" disappearing at times"

The problem was fixed in this changeset, and the fix will be available in the next jetpack release. We'll still beta-testing the new version of Jetpack, but you could grab the Beta version and let me know if the fix works on your end: it is available here:
http://wordpress.org/plugins/jetpack/developers/