you can use google calendar as the medium between your calendar (say sunbird) and your iphone. you never have to touch google calendar again after setting it up, but i'm not sure if you're so against the "google thing" that you don't want to use it at all. if you don't mind "utilizing" it without having to use it, i suggest that setup.
http://www.google.com/support/calendar/bin/answer.py?hl=en&answer=99358
this way, when you make a change in your iphone, it will sync with google calendar OTA (over the air). this will then be pushed to your desktop/laptop Sunbird. The same goes for the opposite. You never have to actually sync your iphone with your computer. the only caveat is that they both be accessible to the internet. you don't have to do anything manually either, no remembering to sync. changes are pushed to google calendar when you make them. you can choose to refresh your calendars (sunbird or iphone calendar) manually or automatically or pushed.
in other words, give Sunbird and Google Calendar a try. if you really dont want to use google as the conduit, i apologize for not giving you a different answer to your question.