Not many, according to the poll I ran on this blog in March. The poll had the largest vote I've had on this blog - 716 were good enough to vote - and 637 (88%) said they wouldn't be signing up for the Prime Minister's inspired mandating of his family values crusade to UK ISPs. Just 47 said they would be signing up.
Of course, much will depend on how the question is phrased when we're asked; if it's "Sign this if you think children accessing pornography is a bad thing" then most will sign not really appreciating that the Family Filter is a nonsense, a clumsy, ill-conceived and ultimately false security measure being foisted upon us all. I hope Virgin Media will phrase the question appropriately.