As some as you may know, Amazon is shipping their print copies of Half-Blood over a month earlier. What that means that if you have pre-ordered Half-Blood, then you should be getting your copy soon.
I'm not sure why Amazon is shipping the copies early, but they have been doing this recently with different pubs and authors.
The kindle version WILL NOT be available until 10/18/2011 as well as other vendors and retailers.
So if you can't wait until 10/18, Amazon may be the place to order your copies.