Sadly, it's not the distance in our case :-(
The problem for us, isn't our distance from the exchange (or at least, I'm pretty sure it isn't - we're less than a mile away), but that as I understand it, our relatively new house (on the outskirts of a large, supposedly high-tech town in southern England) doesn't have the necessary BT fibre connection for us to link to BT Infinity.
In short, we are in the absurd situation where if I check on the BT Web site, it will confirm that yes, our exchange is Infinity-ready, but no, we can't receive the service. I confirmed this with a BT customer service rep - basically, the lines to the houses in our road aren't up to it (copper?), and they had no idea when this might change.
Bit of a bummer, really - we'd be up for an improvement in our Skype call quality...