I've always found "through" to be confusing when used like this. It's like the whole "Next Friday" thing - is that the one about to happen (it's the "next" one to occur), or is it the one after that? (in Australia, it's usually the later)
I much prefer "valid until August 19"
(I'm a native English speaker, not from the US where "through" seems more common)
Right. "Through" is better than "until" because "until" leaves the reader unsure whether it includes August 19th. And 2011 is unnecessary; if someone does find an old message, they can look at the Date: header.