well, I don't have any connections to Nokia, so I can just guess, but in my humble opinion this stuff will not be back-ported to S60 v1.0/2 platform. It probably doesn't have much sense for Nokia to do it if they already have a lot of new phones supporting this (in one of the newest phones N6260 it is already integral part of the phone, at least I believe so).
I'd say it has also something to do with the way how it is written and that it probably needs some features from Symbian OS that are not present on versions before v7.0s
So, it is a challange to write such 'plugin' for DP1.0/1.2.
I'm wondering how long it would take to write a simple version of SIP for Symbian 6.1. There're a lot of such devices on the market.
It is definitely not an easy task. But try to google a little bit through the Internet and/or various symbian developers forums. I believe that there are some guys who already written some SIP stack for S60 v1.x. But I really don't know if they will be willing to share / sell the libraries / code...