Moving Macbook Snow Leopard drive to a Macbook Pro that had Yosemite installed: kernel panic!
Thanks to those who responded to my question, but here's what I ended up doing:


Installed my wife's macbook/Snow Leopard drive in a caddy and attached it to another mac machine as an external drive
Downloaded the OS X Yosemite installer from the App Store and ran it with the external drive as the target
Attached the newly updated external drive to my Macbook Pro as an external drive and booted up off of it (booted up into startup manger mode - press and hold the option key after the powerup tone - and then selected the external drive as the bootup disk
removed the drive from the caddy and installed into the MBP


I'm not sure the third step is really necessary. 

This all resulted in a Macbook Pro running Yosemite with all my wife's data/apps/etc in place with little fuss - my original goal :-)

(If anyone else is following this procedure, a reminder to effectively backup your data before starting! In my case that meant using Carbon Copy Cloner to an external drive.)