Found on Foundry's Nuke Training webpage
IBK Keyer Tutorial - by Steve Wright
Steve Wright, senior visual effects artist, trainer and author, has put together a keying tutorial for Nuke's Image Based Keyer (IBK). If, like me, you've struggled to understand the IBK, you'll find this friendly and informative guide invaluable.
Quicktime Tutorial (87MB)
IBK Tutorial Images ZIP (38MB)