Most of my department now has MacBookPros, most of which have WinXP installed on a second partition for a dual boot setup.
XP runs great! There are two ways to boot into XP, one is to hold down the option (alt) key when the MacBook is first turned on, this brings up a screen with the option to boot into Mac OS X or WinXP. The other way is to use the Startup Disk control panel in either OS X or XP to select which OS to boot by default.
Performance is exactly what you would expect from Intel Core Duo + 667 MHz DDR2 + ATI X1600. When you use the BootCamp utility to add a partition, it burns a drivers CD for you to use in XP. Apple uses a version of ATI's drivers from about the middle of March 2006, so they're fairly recent. Some folks have had success installing newer drivers, some folks have even overclocked the X1600 GPU and GFX RAM via ATITool. Basiclly booting into Windows is just like using any other Wintel notebook.
There are some gotchas. Apple doesn't support WinXP and their current BootCamp utility and drivers are "beta 1" and are lacking some features. There currently are no drivers for the backlit keyboard, automatic screen brightness, built-in webcam, sudden motion gyro sensor, or some of the special keys like volume/brightnes. Screen brightness and volume have to be adjusted with sliders from the WinXP system tray on the task bar. MacBooks only have one mouse button, so you can either download a free utility to let you CTRL-click (like Mac OS X) or you can use an external USB or Bluetooth mouse.
Aside from games, I don't dual boot. Instead I use the totally awesome free beta of Parallels (
www.parallels.com). It lets me use almost any x86 OS from right inside Mac OS X. It uses the Intel VT-x extensions so it's just as fast as dual booting. It doesn't have good graphics drivers, though, so it's not for games, but it's damned fast for everything else.