Well XP has been around for a few years, but has had plenty of updates and improvements. It is now a very reliable operating system with plenty of support from applications, hardware and drivers.
Vista is fairly new (6 months), was shipped before it was finished so still has plenty of bugs, there is a lack of support from applictions, hardware and drivers (particularly for older hardware like printers etc) - although the situation is improving.
But there is no doubt that most of Microsoft's new development will take place on the Vista platform, and some new goodies Microsoft (and others) produce may well be "Vista only".
If you play games some future PC games may be "Vista only" as Microsoft has added some new graphics features that only new games will support.
So it is up to you.
If you just want a laptop for internet, word processing and nothing too fancy then go for XP.
If you want to be "leading edge" and use the latest technology and play the latest games then go for Vista.
Microsoft will probably ship the first service pack for Vista soon and maybe that is a good time to get a Vista laptop, one with the Service Pack already applied.
You MAY have problem getting a laptop with XP on it as Microsoft try to encourage companies to move across to Vista as soon as they can.
Certainly Dell still do XP laptops, and there may be others.