"...Pay $149 extra!" http://www.dailytech.com/Dell+Appeases+Customer+Demands+to+Downgrade+Vista+But+Hits+Them+With+Fee/article12139.htm
Yeah, that $149 extra (actually $148) is $99 to upgrade to a higher level of Vista so that you get the downgrade right in your license, and $49 to have XP Pro pre-installed on your machine. That's a lot cheaper than doing it yourself, as XP Pro still costs more than $100 through the Windows Genuine Advantage program and this way you have two licenses (one Vista, one XP) in case you actually do want to upgrade in the future.
It used to be that the cheapest way to get XP (standalone) was to download a pirated copy, install the Windows Genuine Advantage tool, pretend to be surprised that it's pirated and buy a copy of XP Pro direct from Microsoft under the Windows Genuine Software Offer.
Nice end-run around Microsoft's plans, though...
By the way, that's the American Dell site. And you can still buy XP-only computers for a few more days from there: http://www.dell.com/content/products/features.aspx/winxp_inspndt?c=us&cs=19&l=en&s=dhs
On the Canadian Dell site, there's no mention of stopping offering XP on machines:
http://www1.ca.dell.com/content/topics/segtopic.aspx/winxp_inspndt?c=ca&cs=cadhs1&l=en&s=dhs