The Windows Vista End User License Agreement has this thing in it called "downgrade rights" - well, only the business and ultimate versions do, anyway. These rights basically mean that end-users may purchase the most recent version of Windows, but "continue to run run a previous version until they are ready to move to the new operating system version". The trick employed by HP and Dell entails that they already perform the downgrade themselves, so the customer basically gets an XP machine with an already paid for Windows Vista Business/Ultimate license. This trick only works for XP Professional, as XP Home's license does not have downgrade provisions. There's a cool new term for it too: pre-upgrade machines. The linguist in me loves that term so much.
There are quite a few catches to all this, though. The biggest one I've seen is that Microsoft states in a FAQ about downgrade rights for OEMs:
A. No - downgrade media is provided by the end customer.
Seeing you actually do need an XP Pro license to exercise downgrade rights, I am a bit puzzled by how Dell and HP can do this for you - do you need to mail them your XP Pro license and disk? Does this also mean you cannot use this backdoor to buy an additional computer with a new XP installation, that you plan to use alongside your current installation of XP Pro?
News.com mentions another limitation: seeing customers need to actually request the downgrade to XP, how is this going to work in ordinary retail stores? "While companies can offer pre-downgraded machines via their Web site, things get a little more complicated when it comes to buying a PC at retail stores. It may be possible for customers to buy such a machine, but just how this will work - and if stores will offer such an option - is not totally clear."
As long as customers continue to create enough demand for Windows XP, PC companies will continue to find ways around the end-of-sales date of Windows XP. This is a cat-and-mouse game where it is difficult to predict if either the cat, or the mouse wins. Microsoft is a big player, but its power relies heavily on the willingness of its OEMs. The OEMs, however, are reliant on Microsoft too: people buying computers want Windows.
I repeat what I said last time. If you really want a computer running XP so badly after 30 June, try to buy a computer without an operating system pre-installed, and try to get your hands on a copy of Server 2003, which will remain available for a while. You can use the 180-day free trial to already familiarise yourself with it while trying to find a valid license (I've got 177 days left). It's better than XP, too, in my opinion.