Thanks for confirming the curl setup. The only thing I can figure out right now, based on this article, is that in some cases, the signature appended to requests for assets that redirect to amazon includes encoded strings (like %2F, which is a slash, and %3D, which is an equal sign) that are fouling with the download because the modified value doesn’t match what amazon’s expecting when it validates the signature. I’m trying to sort out a good way to handle this case but am so far having trouble debugging (it’s made harder by the fact that it’s inconsistent — ie, if I include one of your problem files in a Posterous post of my own, it imports ok because I’ve so far gotten lucky with the signatures).