Are USB mains plugs ok for iPads/iPhones?

Yes.

This is why they come with a USB mains wall plug.

What is a "built in USB plugs"?

USB is a standard and it is the same everywhere in the world.

Are you referring to one of the new wall outlets with USB ports for charging devices?

Yes, if it follows the USB standard for charging USB devices mentioned by Chris?

A charger that looks like a mains plug that has a USB port in it? Not a problem, except that many might be pushed to provide enough amperage for the iPad. You need 5 watts for the phone and/or 10 watts for the iPad. Check the specs.

I had not realised wall outlets with USB sockets were available but a quick search on Google shows they are widespread. In the UK, they have to conform to BS standards and the ones I have seen are putting out 5.0V DC at 1000mA.

That's enough to charge an iPhone at full rate and an iPad very slowly.

Newer Technology in the USA offers one that complies with the electrical standard used in most of the Americas and will offer one iPad 10 watts, full charging capacity. If you want to charge an iPad and any other USB device similtenously, they suggest using the AC adapter for the iPad instead.

Thanks for the pointers to USB wall sockets. I'm now going to replace some outlets in my study as I have a plethora of USB charger plugs for various iPods and iPhones.

I don't know about the UK, but the good ones aren't cheap. US$30 in the US, likely more in the rest of the Americas.