The opposite is actually true, in my experience. Many shops use manual regression testing, which is incredibly slow and expensive. Ratios of 1 tester to 2 developers, or even 1:1 or greater are common.
XP, in contrast, has automated regression tests that are created by the programmers. The regression testing burden is much smaller, which reduces the number of testers you need and also frees the testers up to contribute in more sophisticated ways. I've ratios of 1:4, 1:6, and lower.
James Shore, coauthor of <a href="http://www.amazon.com/Art-Agile-Development-James-Shore/dp/0596527675" target="_blank" rel="nofollow">The Art of Agile Development</a>. Website and blog at <a href="http://www.jamesshore.com" target="_blank" rel="nofollow">http://www.jamesshore.com</A> .