Free iPhone 4 8GB on the $30/month plan with Vodafone
They look to be the only provider currently offering no upfront cost on this iPhone model. The next best plan with the iPhone 4 8GB included is with Virgin's $29/month plan where the handset costs an additional $6/month. For the extra $5 monthly spend with Virgin though you'll get an 50MB more of data. But Vodafone's per-minute call rate is slightly cheaper (98c vs 99c) along with the minimum total cost for the 24 month contract period ($720 vs $840).
Still each person has different usage requirements in terms of calls/data/sms, so it's always important to take these into consideration too when looking for the best option.
This offer ends COB on the 2nd July 2013.