In the UK, the rule is that the phone companies just make their plans as confusing as possible. Whether that is intentional, or whether whoever sets up the plans is bad at maths, I cannot say. As an example, O2 sells the iPhone 4GS with six different plans. Plan A costs some amount. Plan B costs a bit more but gives you more; fair enough. With Plans C to F you pay more than Plan A but you get less, so you would have to be a total imbecile to take these plans. Then they have plans where you pay X pound for an iPhone with a 24 month contract, but you can get it for £50 less if you pay £5 more each month for 24 months, total £120, or you get it for £100 less if you pay £10 more for 24 months, total £240.
Not wanting to single out O2, the others are just as bad. And when they have "1 GB data per month", it is absolutely impossible to find out anywhere what happens if you exceed that limit.