Migrating from Old to Less-Old Mac - any other specific issues? Scenario - I have a new Mac, and am giving my old one to my father-in-law.

He now has an old Gx Gumdrop iMac running 10.3 (I think), and the new-to-him mac is an older Core2 iMac running 10.6

From How can I transfer data from a G5 iMac to an i7 iMac? I have an idea of how to connect the two machines.

Other than deleting my old data, etc.; any specific gotchas to watch out for, given that I am targeting not a brand-new mac, but an old one?  Can I just trigger the migration assistant (because normally it is triggered as part of the initial install process)?

Thanks
Yes, if you're fresh installing the new Mac it will ask if you want to transfer information at the end of the install process - just have a firewire to firewire cable handy and have the old mac booted into target disk mode (hold down T while the old mac is booting up). 

Just check you can boot the old mac into target mode first - I know some of those old macs weren't built with the capability. If you can't target mode just comment on the answer or your question, there are other ways around this. 

The only gotcha I can think of is any PPC apps (ones that use the older Power PC processors) like AppleWorks will need to use Rosetta to run, which might make them run a bit slow or cause general funkyness with those apps. You'll want to upgrade your Dad's apps to their Intel versions if you can - using something like App Fresh after the migration would be a good idea.