So I spent the afternoon getting one of those nice shiny iMac 20" loaded up. Nice piece of kit, I could be tempted over the Wintel fence - but then I cheated and loaded the 10.5 Leopard upgrade with Boot Camp and fired up Windows XP Home. It feels quick even with some anti-virus loaded - I used AVG.
So the top tips for loading Boot Camp are, after starting the Boot Camp Assistant from Applications > Utilities and you're about to restart with the XP setup disc, make sure you've got a PC USB keyboard and mouse attached. I used a Dell set I had to hand. The Apple keyboard and mouse from the iMac aren't recognised by the Windows XP installer and you get stuck at the first Press Enter to Continue.
As you're going through the XP install you get an option to format the partition that the Boot Camp Assistant prepared or leave it unchanged. Choosing the unchanged option kept giving me 'disk error' when XP finished the first part of the install and went to restart. There's a note here.
To complicate matters further if you've set the partition to be larger than 32GB it will only format as NTFS. This isn't a problem for Boot Camp or XP but it will restrict you to read-only if you're subsequently viewing the XP partition from OS X. So I went back (again), removed, installed, chose the 32GB button and the install ran through smoothly. Don't forget to insert the Leopard CD once you're in Windows XP and all the relevant Apple drivers will load up.
BTW the Dell keyboard and mouse worked OK with the Mac/OS X so when you get stuck in a loop with the Disk Error, you can use the Alt key at startup to get back to OS X. Also holding the mouse button at startup ejects the CD.