It seems wherever you look now in London you see fixed or single speed bikes it has been suggested that all these riders are copying bike messengers. I think there is more to the trend than simply imitating messengers and to some extent they are many of the same factors that drove bike messengers to ride fixed bikes.
As more and more people commute into London by bike, existing riders who have been commuting for some time want to find a way to differentiate themselves form these hordes of "newbie" riders. Riding fixed is a way of standing out from the crowd of ordinary commuters. It's new and sort of fun/painful. The bikes are super simple to maintain, requiring a lube once in a while and thats about it.
Did messengers lead this trend? Yes they did, they are the lead users amongst urban cyclists. Are other cyclers trying to look like bike messengers? Some but I think the trend is larger than that and encompasses both the practical and the desire to stand out from the crowd.
The interesting thing will be spotting the next trend in cycling.