You ask a good question as I am currently in the same boat. At the moment I Pay £30 a month for 1200 mins and 1000 text + unlimited internet bolt on and using with the Iphone 2G (unlocked). I am hoping that when the 3G version is released it should be as simple as buying one and popping your simplicity Sim in it. Yes i know you wont benefit from the free wifi for 6 months but as mentioned above already get free internet bolt on and wifi from home.
If it doesn't work then I guess the only option would be to wait till an unlock solution comes out and trust me there will be one at some point.
that is what i was going to do
but if you work it out, it costs more than getting it on a £45/month contract and then switching to £35/month after 9 months