"Web application security is still an immature field, and many of the layers are sufficiently poorly designed that issues like this will pop up for a good long while. Just like buffer overflows have been a weak spot for C security as long as the Internet has been around, escaping issues will continue to be a weak spot for web security for as long as we're afflicted with this particular architecture."
It seems like a field not only in its infancy but also oddly unglamorous and under-reported. There's no repository (that I know of, at least) of vulnerability reports of major web apps, for instance, yet it's easy to look up an exhaustive history of Flash vulnerabilities down to the seventeenth decimal sub-version. And yet the various XSS/CSRF/etc vulnerabilities are easily as dangerous and as exploitable. Twitter's dreams of a billion users and a new internet were not exposed by a buffer overflow, after all.