Open source cryptography has the benefit of public scrutiny. If there were back-doors or cryptographically weak code, they would have been called out. It would not be a cryptic announcement.
I've been using dm-crypt since about 2007. (now dm-crypt/LUKS)
See, that's the thing that confuses me, and makes me inclined to believe it *may* be a warrant canary.
But even if they did get served, I'm not sure of:
1. What the NSA would be able to do. As you mentioned, it's an open source cyrpto package - if the NSA was mucking with stuff, people would notice (I think).
2. If the NSA was looking for who all downloaded it, really can't do much about it at this point anyways.