I used ETag in an iPhone app (it was necessary to download a lot of contents, so we used ETag to track the differences) and the result was amazing, worked great

I used ETag in an iPhone app (it was necessary to download a lot of contents, so we used ETag to track the differences) and the result was amazing, worked great and was really easy. If you need use etag for iPhone, I strongly recommend using ASIHTTPRequest library.