It seems to have been around the same time as I upgraded to Empathy here too. Which is worrying seeing that Empathy will be default for a lot of Ubuntu users in the next couple of weeks!
I've disabled my client for the time being but that isn't a solution in the scheme of things!
From the looks of this ticket it accuses Openfire of not correctly implementing the XMPP protocol:
*Pidgin sends XEP-0199 pings as keepalives.
It sounds like you may be encountering the issue where Openfire just completely ignores some IQ packets (in clear violation of the XMPP specs). There are some older tickets relating to that (I don't have the #s handy).
Please file a new issue for this and attach the Help->Debug Window output from a computer being disconnected from openfire.*
I wonder if Empathy/Telepathy has the same issue? I wonder if it is worth reporting to their issue trackers as well?