...we can violate conservation of momentum by invoking some sort of vaguely defined quantum woo
As far as I can see, Roger Shawyer is not a wild-eyed madman, but a serious engineer, who argues his case soberly. That is not to say that his claims are correct, but simply means that he actually offers something that is worth scientific scrutiny and which can be discussed and tested. On the other hand, since this is not all over the new channels, it is not something that has been demonstrated unambiguously enough yet; if this was definitely proven, then we would hear about it even in the general press.
There are two wikipedia articles:
- that seem relatively trustworthy. To me it looks like a slightly speculative concept, but one that could well have some theoretical support; I am not an expert in QM, but to my eyes it looks somewhat plausible. He doesn't claim to violate fundamental laws of physics - if he is right, and this drive works, then it uses the same phenomenon that lies behind the Casimir effect (well, read the articles, really). It doesn't generate energy from nothing, energy is expended in the process; and it doesn't work like a rocket, it seems, but like a paddle steamer, in that it sort of crawls along in the soup of quantum fluctuations (yes, I don't know what actually means either, but it sounds cool, and apparently it is an observed phenomenon, ie. real).