I've recently put in a fair amount of time into researching the whole "Dell/XP/Vista" issue. I've been purchasing Dell PC's for my company for several years. However, I was very close to ending this relationship when I discovered the limited availability of XP on new Dell purchases.
Fortunately, after discussing the issue with my Dell Business rep and scouring the Dell forums for information on the subject, I found a solution.
Microsoft has added an exception to its EULA with regard to Windows XP Professional and Vista Business. It goes something like this: [Until January 2009, purchasers of retail or OEM versions of Vista Business may substitute a previously purchased copy of Windows XP Professional under the same license]. I verified this with Dell.
Of course, in order to make use of this license modification, you have to already own a copy of Win XP Pro.
I've provided a link to the official Microsoft announcement of the license modification below:
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http://download.microsoft.com/download/d/2/3/d23b9533-169d-4996-b198-7b9d3fe15611/downgrade_chart.doc] 
