Hey Mad.

I don't know about home internet, but i have an iphone and i use a pre-paid sim. I topped up $40 when i arrived in Aus recently, 3 and half weeks later, i have used it ALL the time especially the internet, and i haven't ran out yet..... I don't actually ring people much though, mainly because i hate talking on the phone. So lots of short calls.
I bought my phone outright and had a look at the plans V's prepaid and there was no real bonus to having a plan except you can't have voice mail with pre-paid.

I have gone from Telstra to vodaphone to 3 and back to Telstra over the years. I think they are crap but the best of a bad bunch..... Don't get sucked into 3. They don't let you change your plans, are very inflexible and customer service is worse than Telstra.