I confess that I've managed to miss hearing/seeing much about this one 'til you mentioned it here, MGM. I wonder if the reason you've not had much response is that -- even if folks have heard of the case -- they (like me) don't recognise Mr Tappin's name. Maybe the thread would attract more attention with a Subject along the lines of "Another unfair US extradition of UK citizen"?

I read the Daily Mail article you mentioned above, and read a couple more after Googling the subject:

Telegraph: David Cameron not sympathetic to plight of alleged arms dealer
and
Forbes: U.K.'s Chris Tappin, U.S. Government Informant or Enemy? (I wanted to find an article with a US view as well.)

As a USian living in the UK, I all too often find my government's behaviour appalling, outrageous and seriously upsetting. I agree with British opinion that this extradition agreement with the US is absurd, and can't really see why the UK govt 1) agreed to it in the first place, and 2) hasn't repealed or at least modified it by now. Why on earth agree to let another country extradite one's citizens without at least being granted the same courtesy in return?

I have particularly been concerned about the plight of
Gary McKinnon (here's a link for anyone (prolly Yanks) who hasn't already heard of him) (especially since, if I bring up the subject when I'm visiting the US, it seems almost no-one has even heard of him). It is harder (for me) to feel quite the same amount of sympathy for Mr Tappin, but I agree the principles still (should) apply to him as well, and that it is completely unjust that the US apparently has been allowed to extradite him (in handcuffs!) without showing evidence, while his own witnesses won't even be able to testify on his behalf.

I'm also concerned about Mr Tappin's wife; according to the Mail, she has Churg-Strauss syndrome (which is what killed UK DJ Charlie Gillett) and he has been taking care of her. What will happen to her if he's locked up in the US for the rest of his life?

It is absolutely sickening.
But since what he's accused of is -- in effect -- (a form of) arms dealing, I don't foresee Avaaz or Amnesty International championing his cause anytime soon...