Seems fairly transparent: members of the team would like to make it open source closer to the release, but they haven't made a decision yet and will likely do so once the 1.0 release is complete.
I did phrase that terribly. What I meant (and failed) to convey was that the most authoritative source of info on this topic was a post to a relatively obscure LLVM-related mailing list, rather than any sort of official announcement. However, I shouldn't have said "lack of transparency", because the devs could have chosen to not make any sort of public statement at all.
Considering that Swift is built on LLVM, it's not surprising that a comment was made there. It's also probably where an announcement of it being open source would be made (and now probably the Swift blog).
I've heard from various people that any decision on open source wouldn't happen until after 1.0 since the language syntax decisions haven't been nailed down yet.