Credit scores are irrelevant to FHA insured loans. The only guideance that FHA provides on scores is that if a borrower has at leats one credit score, the application must be run through FHA's automated underwriting system (AUS) known as FHA Total Scorecard.
However, lenders are free to add their own restrictions on top of FHA's guidelines. Most lenders have set a minimum credit score of 580 for FHA insured loans regardless of AUS results. This is driven by investors - primarily Bank of America/Countrywide, which provide lines of credit to many small and medium lenders.
Some lenders, however, will fund FHA insured loans for scores below 580. Flgatstar Bank on the retail side will accept scores as low as 500.
Bear in mind, however, that the score is not the issue with underwriting an FHA insured loan - the content of the credit report is the key. A low score often indicates a borrower may have too many negative tradelines to qualify for an FHA insured loans unless substantial compensating factors are present - a large down payment and/or substantial cash reserves after closing are often used to compensate for poor credit.