I recently got the Jelly Bean update (4.1.2) for my Samsung Galaxy S2 (i9100) and today I was pleasantly surprised to see that the native SIP client is enabled on the phone now. I previously used SipDroid then after upgrading to Ice Cream Sandwich I had to use CSipSimple when in 3g/HSPA+ networks.

To get this set up you just get into the native dialer settings and at the bottom are the Internet Call Settings.

Now I just need to see if battery life improves or deteriorates with the native SIP client.