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