There are quite a number of wedding photographers here in the Philippines who use Pentax gear, and I'm sure there are even more in other countries. It'll boil down to personal preference, current investment in a system (like you, for example, if you have tons of good Nikon glass already, why switch?), and, of course, personal talent.
You can look up certain personas in this forum, as there are established professionals who drop in here from time to time to join in the discussion. Good folk, really. Of course, the biggest name being Benjamin Kanarek (benjikan here in the forums), a well-esteemed high fashion photographer who's sponsored by Pentax.
I'm sure there are other regular posters here who are professionals, but I cannot ascertain who they are as most people here are humble keep a low profile, blending in with the hobbyists and common folk like me. Marc (35mmfilm_user), I believe, is also a working pro using Pentax. Chris (codiac2006) works at a big photo store, but I also believe he does paid photography as well.
Guys, care to chip in some more forum members who are working pros using Pentax?
Me, I'm considering trying my hand at making money out of my photos. Not sure if I'll succeed, though, but I'll keep trying and will be using my Pentax equipment.
Like what you'll usually hear in this forums, the camera is just a tool. It's the photographer who gets paid. So, essentially, don't be put off by the brand and the camera model. It wasn't too long ago when photographers used 4 MP cameras and still got paid lots.