Max, I recently picked up Dr Oppenheimer's book. I'm still reading it, but I'll be the first to admit he's got some kind of axe to grind. He's seriously into Celtoscepticism. On the language side of things, he's using results from Prof. Isidore Dyen at U of Hawaii. Dyen is into lexico-statistics, which is kind of a rewarmed glottochronology. Though there is at least one other historical linguist, Donald Ringe at UPenn, doing similar work in what is going by the name of cladistics. More later.

I also noticed two entries over on Language Log that take him to task: this and that.

Ceci n'est pas un seing.