While the following two lines are equivalent in OS X 10.4 Tiger:

Applescript:

(current date) as «class isot» as text
(current date) as «class isot» as string

in 10.5 Leopard the first line generates an error that states "Can’t make [data] into type Unicode text."  This, despite the fact that both text and string were moved to unicode text in Applescript 2.0.

Thanks to Bruce Phillips for finding this one!