[derf_]

That is a common misunderstanding of first-to-file. Under first-to-file, prior art is absolutely still a defense, and much easier to prove than under first-to-invent. If you publicly disclose something before someone who invented, but did not disclose, the same thing, your publication counts as prior art. It is much easier (read: cheaper) to demonstrate that "X document was published on Y website on Z date" than it is to start doing discovery to compare private lab notebooks or whatever to decide who really invented the thing first.

First-to-file is mostly about simplifying the complicated litigation edge cases where two people claim to have invented, but not disclosed, the same thing during the (US-specific) one-year grace period they have to file a patent on it.

Prior art may still not invalidate the patent, though. Usually the patent holder will argue that their patent differs from the prior art in some respect (you can often find these arguments in the patent's file wrapper at http://patentcenter.uspto.gov/). But that narrows the scope of the patent, and being your own prior art is a pretty good defense against infringement.