Unless you organize something before leaving the UK, using your mobile in North America could leave you penniless (as others have commented).
Unless you're planning on a many-month excursion (and deem your iPod repertoire essential), just listen to "local" radio in your car rental, which might also have as a perk satellite radio.
Last but not least: Any time you enter the good ol' USA with electronic devices, border security (by whatever name) has the authority to examine in detail everything you've got on your devices (eg, laptops, iPods, iPads) and/or confiscate same. So, delete anything which paranoid Uncle Sam might construe as a homeland threat or porno or ... OR just don't travel with them. It's not a pretty picture.
thanks for all these heads-up, I have now examined the Vodafone website wrt all this. I have to tell them that both phones are being taken out of Europe, ok. My phone is so old that it won't browse the internet anyway; the other one is not going to be used for that, I have just decided. It would be stupid to go so far from home without emergency communication devices, so we'll take them anyway but only use in emergencies.
Any calls back to Europe will be on landlines and very short indeed........
As to border security potentially confiscating my iPod, I'm not going to be scared by that. It only has music on it, and I'll need it for both long-haul flights. If people were scared so easily no one would take any i-thing on any flight and AFAICS planes are full of them.
