I think GCSEs are becoming increasingly used as a means to distinguish candidates with otherwise near-perfect UCAS applications. If you've got two candidates, both with all A's at AS, both with really good personal statements and references, and both with 3/4A predictions at A2, the only thing left to look at is GCSE performance.
I suppose it was one of the reasons for the introduction of the LNAT (although I'm very dubious about how useful it actually is). I have heard, however, by word of mouth, through admissions tutors and fellow applicants, that some unis (the ones currently doing the rounds are LSE, Manchester, Warwick in particular) have a nominal 'benchmark' that most of their successful applicants have to achieve at GCSE. As evidence to back this up, I've seen the record of applications for September 2005 at my college. Around 11 people applied for Warwick for law, and only 2 got in. Fair enough, you might say, but when you see that both of those applicants got at least 6A*s at GCSE and all those below them were rejected, it does look a tad suspicious...