Have you had the SIM replaced? The dot suggests a basic signal but no 3G signal. I live in Staffs, and have patchy 3G coverage but it's not a problem at home as I have wifi.
I have to say, some of my friends have the iphone too, and no-one has mentioned a similar problem, and if it's 3 replacement handsets maybe the sim needs replacing too?
PS: if you need the SIM replacing just call O2 they will send a new one out.