Most likely the same root issue as this: http://wordpress.org/support/topic/fatal-error-call-to-undefined-function-jetpack_is_mobile?replies=10#post-3208065
I've reproduced the bug, and until Jetpack 1.8.1 comes out, you can "fix" the problem by enabling the Subscriptions module.
If you already had the Subscriptions module enabled, let me know.
This bug has been fixed in Jetpack 1.8.1, which is now available at http://wordpress.org/extend/plugins/jetpack/
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