On Dell's page for the XPS 15z: "Just 0.97" (24.68mm) at its thickest point, XPS 15z is the thinnest 15" PC on the planet". Clicking the fine print, it says, "Based on a Dell internal analysis as of May, 2011."
Fair enough. But a different version of the fine print, found by Charles Arthur at The Guardian is a little more striking:
Based on Dell internal analysis as at February 2011. Based on a thickness comparison (front and rear measurements) of other 15" laptop PCs manufactured by HP, Acer, Toshiba, Asus, Lenovo, Samsung, Sony, MSI. No comparison made with Apple or other manufacturers not listed.[emphasis mine]
Explaining why Engadget noted in their review, "it's actually a few hairs thicker than a 15-inch MacBook Pro."