One of the important things about earning your living from photography is that you make the decision on what to use based on how well it will get the job done in any given situation.
If m4/3 does the job for you then use it, if not use something else. If people pay you they expect results, and they expect those results to be consistently good, what they are looking for and worth the money they paid you to get them. If you know, and I mean really know, that m4/3 can do that then there's no problem.
As other photographers have pointed out, going back a few years we were using cameras that are nowhere near as good in terms of IQ, speed and usability as current m4/3 cameras. Indeed I was shooting weddings and studio portraits with a Fuji S2 Pro in 2003/4 as were many of my colleagues. A GH2 is a vastly superior camera to that in almost every way.
The most important thing isn't what gear you use but how well you know it and can get the best results from it. You also have to be sure that it can do the job, you can operate it so that it can do the job, its reliable enough to do the job and the results will justify your fee.
If you think it might do the job then thats not good enough. When I use m4/3 for "pro" jobs I am sure it can handle the demands of those jobs because I've tested, I've used it before etc. If I have a job where I'm unsure if it will do it or not I use something else that I'm 100% sure of. Even then I'll make sure I have backups for everything.
Being professional is more about what's in your head rather than what's in your bag."