I upgraded 2 hosts last night from 5.1 to 5.5 (yeah I know...what's the rush?) and after the upgrade, the console tab in the vSphere client stopped working. It no longer shows any consoles of active VMs. The good news is the "Launch Virtual Machine Console' icon does still work, so this is more of an annoyance than a major problem.
I am using the vSphere client 5.5.0 Build 1993072 and vCenter Server version 5.5.0 Build 2001466. Upgrading the VMWare Tools in the VMs had no effect.