Yes, Windows XP is still available on brand new computers from major retail outlet. The official line from Microsoft was that June 30, 2008, ended the era of Desktop and Laptop PCs shipped with the XP operating system. Also, Microsoft announced in April 2008 that it will extend the use of a type of XP for Ultra Small PCs (these are tiny laptops that use an "Atom" processor). However, XP it is still available from Microsoft for many new computers.
I have looked at many large computer retailer web sites. One site I checked had no less than 38 Desktop PCs and 23 Laptops being shipped with "Downgraded XP Pro" and sometimes with Vista - so that you can choose which operating system you want to install. Clearly, Microsoft has back peddled in deference to die hard XP users: It's a good thing.
Why Would You Want XP Now?
Why would you want to buy a new computer with XP on it? Good question. Well for one thing, you will not have to upgrade any of your current XP applications - that's a potentially big money saver, especially in this economy. If you are already familiar with XP, then you will not have to learn Vista. Also, you won't face any compatibility or driver issues between Vista on your new system and XP on your old. And last, but definitely not least, XP is tested and proven; Vista is still a bit unpredictable.
Are Their Any Downsides to Buying XP Now?
Downsides are often in the eye of the beholder. Technically, there are no downsides until Microsoft stops supporting XP in 2014. Also, hardware and software manufacturers usually ship new products that are compatible with older versions of Windows operating systems (check the system requirements to make sure).
Bottom Line - It's what you want that matters
If Vista has new features or functionality you must have, get Vista. If you'd like to keep going with XP, you still can.