I think many of the other commenters have done an admirable job defending Putnam’s usage of thought experiments, so I don’t feel a need to address that.
However, there also seems to be some confusion about Putnam’s conclusion that “meaning ain’t in the head.” It seems to me that this confusion can be resolved by disambiguating the meaning of ‘meaning’. ‘Meaning’ can refer to either the extension (i.e. referent) of a concept or its intension (a function from the context and circumstance of a concept’s usage to its extension). The extension clearly “ain’t in the head” but the intension is.
The Stanford Encyclopedia of Philosophy article on Two-Dimensional Semantics has a good explanation of my usage of the terms ‘intension’ and ‘extension’. Incidentally, as someone with a lot of background in academic philosophy, I think making two-dimensional semantics a part of LessWrong’s common background knowledge would greatly improve the level of philosophical discussion here as well as reduce the inferential distance between LessWrong and academic philosophers.
Norbert Wiener