Today I found out that in vSphere 5.1 the SSO administrator account (admin@system-domain) has a password that expires after 365 days. See KB2035864:
vCenter Single Sign-On account (SSO) passwords expire after 365 days, including the password for admin@system-domain.
In vSphere 5.5 it gets even better – the password expires every 90 days by default! (See the vSphere 5.5 SSO documentation)
By default, vCenter Single Sign-On passwords, including the password for administrator@vsphere.