I notice battery life is poor when SIP client is registered for long periods (which is probably a bug).
I have set client to not register automatically; only when making internet call does the phone register. But once I have made an internet call the phone stays registered. I would like it to unregisted after making a call. Only way I have found to do this is to power off then on again, which is a bit slow.
On my E61, there is an application named Internet Telephone which lets you register and unregister the SIP client. I suspect it originated in the Connectivity folder or the Tools folder (I've since shuffled stuff around)