Hi friends,
For your information, Robert, Nicola and me had a closer (phenomenological) look into the ratios of B->Dh decays, in the light of the determination of fs/fd.
In case you are interested, we just submitted a preprint describing our findings: http://arxiv.org/abs/1012.2784
In summary, we have reached the following conclusions: 1) If we use the ratio N(B0->D-K+)/N(Bs->Ds-pi+) to determine fs/fd we argue that the uncertainty from SU(3) breaking of factorization is 2%. 2) We propose that we can also use the Cabibbo favoured decay, ie. N(B0->D-pi+)/N(Bs->Ds-pi+), for the determination of fs/fd. The price to pay is that the theoretical uncertainty is a bit larger due to the extra ("exchange") diagram in B0->D-pi+.
Cheers, Robert, Nicola & Niels