Usually, when I come to the forum while thinking that the question is not 100% acccurate, I encounter lots of similar remakrs already. Today, nobody has yet raised the point I want to make - so I'm beginning to doubt myself. Please tell me if I misread Books Online...
On the page referenced from the question's explanation, it says:
For more information about required SET option settings with indexed views and indexes on computed columns, see "Considerations When You Use the SET Statements" in SET (Transact-SQL).
Following that page and scrolling down to the last paragraph, I read:
If any one of these options is not set to the required values, (...) SQL Server will process SELECT statements on these tables or indexed views as if the indexes on computed columns or on the views do not exist.
My interpretation of this text is that indexes on computed columns will not be used when ANSI_PADDING and various other SET options are not set to the required value. But the explanation in the question says the exact reverse: "(...) but existing indexes can be used" (emphasis is mine).
Did I misinterpret the documentation? Or is Tom incorrect? (Christian's test code suggests the latter).