I haven't posted on this issue recently (although I'm working on a column for TechCentral Station,) but Dr. Manhattan has been writing some good pieces on it (check the comments sections for more.)

There's one thing I'd like to add, as an organic chemist. A persistant statement that I see about thimerosal is that it's "49% ethylmercury by weight." This makes it sound like it's a mixture of components, one of which is ethyl mercury, which really isn't the case. Thimerosal is a single compound, a mercury with an ethyl group and a thiosalicylate (the mercury is in the +2 oxidation state.) When sulfur is bonded to mercury, its reactive characteristics change quite a bit. A dose of thimerosal would not be expected to be the same as a dose of ethyl (more correctly, diethyl) mercury. And it isn't.