Today I downloaded and ran the free Linotype FontExplorer X font utility and Tiger Cache Cleaner in demo mode. Between the two of them, my OS X 10.4.7 problems have gone away and iData 2 is useable again. Hooray!

FontExplorer X and Tiger Cache Cleaner cured the problem I was having with Apple Mail as well as a strange double-spacing problem with TextEdit and a text-entry captioning problem in Photo Mechanic.

I'm a happy camper! Now I'm off to purchase Tiger Cache Cleaner which seems like a great product.

Thanks to all those who responded to my previous message; you helped me realize the problem with my machine and not a wider installation problem.