Commercially, this process is known as 'grey importing' but it is perfectly legal. Because the commercially legal status of grey importing is a somewhat grey area (hence the name) what we're advising here is how to import an iPad for personal use. While it's by no means illegal to take products from one part of the world to another, some companies have cracked down quite heavily on grey imports in the past and you need to be careful of tax and import duty issues.
Most companies - including Apple - have taken a largely neutral stance on the issue. We presume because Apple largely secures a worldwide launch for its products, with a notable exception of the iPhone (because of contract negotiations with carriers and governmental approval issues that surround mobile phone devices) and now the iPad (because it's finding it hard to meet demand in the USA). This is the first time most Apple fans have ever had to seriously consider importing Apple products from the US.
Incidentally, if you think that importing iPads from the USA to the UK is going to be a money-spinner, you might want to do your maths first. There are better ways to make a living.
Apple will currently only ship iPads from the Apple US store to legitimate US addresses and is limiting orders to two iPads per customer. So you can't just purchase an iPad from the Apple Store in the US and get it sent to the UK, you'll need to get it delivered to a US address. Also, while Apple accepts international credit cards, it only enables billing to a US address. And your credit card is mostly likely to have a UK billing address. This disparity means that your credit card company may decline the payment.
So you need to buy an iPad from Apple or an authorised US reseller, and get it shipped to a US address, and then get it forwarded from the USA to the UK. For this you will either need a friend in America, and a means to give them the cash. Or you need to investigate the use of a redirect service such as BundleBox.
When it comes to buying an iPad then the best place to go is Apple itself. Although they are shipping in 5-7 days in the US now because of a backlog of orders there. You can place an order on the Apple store in the US using a UK credit card, but because you have to enter a US address (your friend's house) it may be declined. In which case a phone call to your credit card company may do the trick (or it may not). If the credit card company approves the payment, Apple should ship the iPad to the US address.
If you have a friend in the US it's probably better to use a service such as PayPal to send them the money for the iPad and shipping and get them to buy it and send it to you. They may be able to pick one up from an Apple Store, although our understanding is that Apple is currently sold out at stores for the next week or so.