When I came home, I did some googling, thinking it could be Skype, or it could be another app. I came across a Skype forum post where someone said they lost 70% in four hours, just like me. There were no other apps running that would have been keeping a connection open, so it's safe to say - given the vast number of people who have commented on the relationship between Skype and poor battery life - that there is a link.
So, if you use Skype, and see bad battery life on your iPhone, try quitting the app and see if this improves things. I haven't done any scientific testing, but there seem to be enough people who have this problem to suggest that Skype may be the culprit.

