Hope I understand the question; I have been using extensively version 0.7 (that's the latest) for several months in my N800 without serious problems. If you want to build something on top, I think that's the version to choose. I would be happy to test anything you build.

Some side notes:
1. The SMS is not sent if the phone is inside a menu (maybe this is a specific problem on my mobile, Nokia 6310) and you don't get any notification about it. It has happened to me many times to be inside the phone's contacts menu reading the number of the person I want to sent the SMS, which meant that I never sent that message. Very annoying..
2. There isn't a confirmation that the SMS was sent.
3. The delivery notification (you can set in your phone's SMS settings) never arrives if you sent it through the phonelink software.
4. The sent SMS is not saved anywhere (neither on the software nor in the phone's memory)
5. It uses something that comes with the GPE (can't remember now), but it is not registered properly (sorry for missing the proper technical terms). What I mean is that you can uninstall it without getting the prompt that this is used by the phonelink. Thereafter, the phonelink fails to start without any details, unless you start it in xterm witch tells you which part is missing.

Big thanks to Fred for his efforts.

Chris