It does seem to have become a bit of a richer person's sport if you go by the STW regular's at least (refering to the "What do you earn" thread a week or so ago that revealed wages on here are generally WAY above national average).
But then it is quite an expensive sport if you're going to do it to the point where you push the boundaries on your equipment. I mean, most of us consider £500 a very cheap mountain bike these days. Compared to the sport which used to be seen as the preserve of the rich (and is now anything but), Golf, where £500 buys you a top of the line set of golf clubs now as prices of Golf equipment has come down dramatically over the years. And you can't argue that Golf costs loads to play in comparison to mountain biking, cos how much do we all spend on petrol driving to various trail centres each year?
Certainly, of the kids I've known into Mountain Biking, the ones that have stuck with it at all have almost without exception, had rich parents that have helped out financially with the sport to some degree.
And of course there's the usual "you could've bought a car for that" response from people if they don't ride that ask you how much your bike cost.
Of course it's not exclusively for the rich, but because it's not a cheap sport, primarily that's going to exclude a lot of less well off people from getting involved with it in the first place as it would represent far too large an investment for a "hobby." To make matters worse, the rapid devaluation of the £ isn't helping right now as it's pushing the prices of everything up, further alienating the less well off from trying it!
Now I love the sport, have done since I started as a 14 year old kid. I will admit though that my Dad did buy me my first bike for Christmas when I was 14, though I never recieved a penny more towards the hobby after said bike was bought. But I was desperately saving up for a bike and buying MBUK every month for 6 months or so, before my Dad stepped in and bought me the bike for Christmas, so undoubtedly I'd have tried it anyway.
There are exceptions to the rule, but certainly most people that I know that ride, and most I come across that do, are reasonably well off themselves, or have had financial support from parents when starting at an early age. Much as I'd like it not to be the case, and that MTBing really was a sport for all classes, sadly due mainly to economic reasons it isn't!
And like Golf used to be, it's now seen as the "socially mobile" sport to be seen doing by many people. Though of course there are those of us that have been riding a long time, and just love riding and don't care what anyone else thinks! 
