I see it was included in the October 2009 release, referred to as “More flexibility for list literals and subtyping” (although it also includes arrays) in the release notes.

For some reason *cough* you seem to be using Beta 1 – the latest and greatest F# does upcast implicitly in list and array literals. So that upcast is in fact no longer necessary, and even results in a compile error.

Cheers, Kurt