The school I'm at now wanted 6 A*-C grades incl. English & Maths, with a minimum of C in the subject you want to do at A-Level if you did it for GCSE. A couple of people got in with D's in Maths & have just retaken it.
Quite low I know! For the yr11s starting Sep 09, they need 7 A*-C passes.
5A*-C which seems to be the case for most state sixth-forms, colleges. Although, having said that, there are people at my sixth form doing a BTEC with no GCSEs at all (i.e. all less than a C grade)
My school doesn't have any requirements to get it, but you do have to resit GCSE Maths and/or English if you got below a C. You also had to get at least a B in the relevant subject to do French, German or Biology, and an A to do Physics or Chemistry. Plus, for most subjects you had to pass at GCSE (there were some exceptions eg. You could do History without taking it at GCSE, if you got an A in English.)