A pro camera is what you can make money on.
It's kind of about knowing what you need, too: with the kind of work I do (And the halting pace at which I can do it) it wouldn't make a lot of sense for me to lay out for a multi-thousand dollar system, for instance: the nice thing about Pentax is, you can have something like a K20d for the price of someone else's stuff that's oriented toward less-serious photographers: you can have your full controls and weathersealing and as good of glass as you can afford: for me, the service network would be of less importance, too: it'd make more sense for me to simply get a backup if I were relying on the camera for pay. I'm pretty much about basics, so I don't need high FPS performance and the like, so if there's a limited selection of really long glass, that doesn't bother me. As long as what I do need exists, we're good.
The Pentax flash system certainly leaves a lot to be desired, but I was always an automation resister, anyway: once upon a time, I never used a dedicated flash to begin with. I walked into that with my eyes open, if without great enthusiasm on that count: I'm not as quick on the calculator dial as I used to be, so there'd be a lot of appeal if they could work some true TTL in.
It used to be that the 'pro' cameras weren't even the ones with all the highest specifications and the most high-tech to them: 'pro' meant it was durable and had the best viewfinder. For a while now, Nikon and Canon have been pretty much defining what a 'pro' camera is in the first place: what's not apparently been the priority to them is something like what Pentax'll give you: something smallish and modular with a nice finder, with good build quality, proper manual controls, your weathersealing, and suchlike.
Knowing the right tool for the right job is one of those 'professional' skills. Personally, if I bootstrap together my enterprise here and find my performance or other practical needs exceed what Pentax can provide, well, I'm not out any more than I would have been if I'd bought 'amateur' grade stuff from Nikon or Canon.
The prices of everybody's stuff have gone up since then, but when I got a chance to go digital, my own math meant that I'd have spent as much to be working with a used d70, and just about all I'd be spending to get that system going would be spent on stuff I'd be guaranteed to be wanting to upgrade and having to live with the meanwhile, anyway, so it wouldn't really be helping me or saving any money to start my digital off with that system. When all's said and done in getting what I need in Pentax, it'll have all come in for about the price of a d300 body, (as it was then, anyway: out of reach, anyway) which would have been what I'd consider 'sufficient for the forseeable future.'
Which is a long way of saying, if I end up needing (and having the budget for) more, it wouldn't be any more 'professional' a move to be trading in entry level Nikon stuff than if I had to trade in Pentax stuff. (not something I'm in a hurry to do, actually. *cling.*
) And in the meantime I wouldn't have been shooting what I wanted to be shooting.
But it all really depends on what you're doing.