I am in a situation much like yours.
I am a long time O2 customer and as such over the years I have haggled down my tariff to a price that I am very happy with. I wanted an iPhone and had 2 options.
1. Wait for the UK iPhone and accept an O2 contract that is almost double the monthly cost for almost half the amount of minutes/text and fork out £270 for the phone itself.
2. Buy a hacked US iPhone, keep my current tariff and perhaps sell any handset O2 provide me with when I renew my contract to help reduce the cost of the hacked handset.
I went for option #2. My iPhone arrived last week and despite a few flaws, I am in love with it.
Now I have a couple question for other UK O2 customers using a US iPhone.
GPRS/Edge/WiFi - You can pay £7 for a bolt-on that will give unlimited data access right? sounds like a good deal when Edge becomes available (it might be already, I dunno) but what is the unlimited WiFi all about? does it give you access in public hotspots? and is that included in that £7 bolt on?
I spoke to someone at O2 who couldnt tell me what the WiFi was, they also told me that it is for iPhone customers only and not availble seperatly for other customers. Is this correct?