Birth Control and Antibiotics
These are some good very recent (2012) pieces of information from the Faculty of Sexual & Reproductive Healthcare (they apply to combined oral contraceptives, but the same source says that it applies to other combination methods and progestogen-only methods too (except the hormonal IUD and the shot)):
"Recommendations on antibiotics have changed since publication of previous Faculty guidance on Drug Interactions with Hormonal Contraception (2005) and the UK Medical Eligibility Criteria for Contraceptive Use (UKMEC) 2009. In line with the World Health Organization (WHO) and U.S. Medical Eligibility Criteria for Contraceptive Use, 20104 the CEU no longer advises that additional precautions are required when using combined hormonal contraception (CHC) with antibiotics that are not enzyme inducers. Minor changes have been made to recommendations on concomitant use of enzyme-inducing rifamycins (such as rifabutin and rifampicin) and CHC."
"Additional contraceptive precautions are not required during or after courses of antibiotics that do not induce enzymes."
"Overall the evidence does not generally support reduced COC efficacy with non-enzyme inducing antibiotics. UKMEC 2009 advises additional precautions with antibiotics that do not induce enzymes. This was an interim measure until evidence could be reviewed in detail by the CEU and other UK organisations. Having reviewed the available evidence, the CEU no longer advises that additional precautions are required to maintain contraceptive efficacy when using antibiotics that are not enzyme inducers with combined hormonal methods for durations of 3 weeks or less. The only proviso would be that if the antibiotics (and/or the illness) caused vomiting or diarrhoea, then the usual additional precautions relating to these conditions should be observed"
So in short, general antibiotics DO NOT affect birth control, aside from side effects like vommiting and diarrhoea in oral birth control.
Rifabutin, Rifampicin and their generics CAN affect all birth control as they are enzyme inducing.