Dell informs us they will quit shipping Windows XP systems after October 22, 2010:
Per Microsoft guidelines regarding Windows XP, system vendors like Dell will no longer be able to ship systems with XP Professional and XP Home after October 22, 2010. This means that we will stop offering XP as an option for customers starting this month in preparation for next month's deadline. One other important thing to note is that Dell plans to continue Windows XP driver support until December 2012.
After the October 22 date, qualified customers will still be able to get systems with Windows XP (more details below) through Dell's Custom Factory Integration service. Otherwise, customers who order new machines with Windows 7 Professional or higher can run XP applications in Windows XP Mode. Click on this link (or the image below) and choose the Windows XP Mode tab like I did in this screen grab for more details. And here's more from Microsoft on Windows XP Mode.