Nexus S free on Vodafone $29 cap plan ($180 call value, 200MB data included)
If you sign up on line, you get $50 credited to your bill I think on the 3rd bill. Plus the $84 cash back, it makes the minimum cost for this phone over 2 years $562.
the only fault I can find for this plan is the 200MB data, this can be overcome by the $4 classic data pack which add an extra 700MB. This data pack may be a 12 month commitment, so make sure you read your terms and conditions