There’s a nice tool that will perform SSL man in the middle attacks. In layman’s terms this means when this tool is run on a gateway it will create a clear text HTTP stream on the network (that you can sniff) when someone creates a HTTPS session. i.e. Someone logs into Gmail via HTTPS and there will be an HTTP clear text mirror of that session which you can sniff via Wireshark.
Check it out here: www.thoughtcrime.org/software/sslstrip/