-PLUS- I can assure you, I would most certainly stay below the ¥1695 ceiling if and when I get the iPhone because I would not use the data at all. I would only access the internet with wifi and use it as a phone/camera.
When I'm home, I'm using wifi, but most of the time I'm on the train or in a coffee shop (when will Japan jump on the "free wifi in public locations" bandwagon? Never?)... so I rely on 3G for probably 70% of my usage.
I switched from AU to Softbank to get the iPhone and was using a similar tiered data plan.... I was only using email and WAP internet and was maxing out each and every month...