Ben and Iain,
Many fisheries down south are now banning BARBLESS hooks. It has been found that on many fish, but mainly Carp, Tench, Bream, Chub and Barbel that barbless hooks can tear through the flesh on a hard fight. The barbed hook holds in one position far better and resists tearing during the twists of a fight.
Trout are interesting as there mouths are a little different in that they have a hard outer but relatively soft inner. Certainly, for Pike, I use Barbless due to the boney nature of the jaw and little risk of tearing.
Where barbless do come into there own is if a fish is deeply hooked, for very obvious reasons. I have used both barbed, barbless and de-barbed hooks for trout and I have found no difference at all to tell the truth...but then I only use very small hooks for trout. A large, rank barb on a size 2 long shank lure hook is always going to be more of a problem.
One thing that has always concerned me was the use of barbes doubles and trebles on Salmon rivers when Parr have been a problem. Salmon hooks tend to have a rather large barb and times that by two or three and you potentially have an item that could seriously damage a Parr even if the angler was careful. Certain times of the year I would like to see single, barbless hooks for salmon on the rivers to protect the Parr.