Ken Burns is awesome, his namesake iMovie effect is not. If you want to do these types of pans with far more smoothness, options and power, Get "Photo to Movie" -
http://lqgraphics.com/software/phototomovie.html
Gee, when I bought it, it was only $10! Now it is $29.95. Still, not a bad price for a program that does what it is supposed to perfectly. Apple really should have just paid these guys to include this in iMovie - they did it really well.
If you don't know much about Ken Burns, you should probably see his Civil War series (rent it from your local library). Even if you are not a Civil War buff (I'm not) it is an awesome and powerful film. Though not his first documentary, it is his most famous (his brother also worked with him on almost his films). Any way, Ken is known for historical documentaries on American history. His films are known for their use of still pictures and narration - this style has been copied by a million lesser documentaries on The History, Discovery... channel, but when you see the real thing, it is so much better.
P.S. The whole reason I came to this post is my sister in law (who I convinced to switch from a PC to a Mac and ended up loving her Mac) is now having problems where iMovie is crashing as soon as it starts up. I did a search and found nothing on it here. She has done the following with no luck:
Downloaded the latest version of iMovie
Downloaded the latest Mac OS
Downloaded the latest QuickTime
Repaired Permissions
Checked for third-party iMovie Plug-ins (none)
She has an 800 mhz iMac G4. I had the same kind of problem with iPhoto and repairing permissions did the trick. Any ideas or help is appreciated.