nowadays, "pro" cams and non-pro cams make no difference to the
non-pro.
in other words, get (and pay) for a "pro" cam only if you can USE
the features found on a "pro" cam that you can't find on a non-pro
cam.
just to list a few, there are the pro cams that i know of (that are
currently in production):
Nikon:
D2x ($7k)
D2Hs ($6k)
Canon:
1Ds2 ($7k)
1D2n ($13k)
this is for 135 format (actually not even, most of the cams above
still use APS sized sensors ie, smaller.)
these are the main differences between pro and non-pro cams:
1. Faster AF and AF tracking. most shooters can't tell the
difference between a pro and non-pro cam, with regard to auto focus
speed. for objects that don't move, u can slowly take your time to
focus on it. for objects that move, u need the speed. and for
people that NEED to make sure watever that's moving fast is in
focus, are generally sport shooters. AF tracking refers to LOCKING
on a moving object so that it continually focuses as you are
framing. for example, a 100m sprinter running towards you, or an F1
car coming towards you.
2. Weather sealed body, all metal construction. If you need to go
to the middle east to shoot war, or the amazon jungle for nat geo
then this is a must, otherwise most bodies, pro or not, can tahan A
LITTLE rain, or humidity, or dust. and the all metal body allows it
to take knocks and drops, in CASE it does. to a pro, getting the
shot is no 1, his equipment can take a few knocks, or rather the
equipment HAS to, and still be able to perform.
3. High frame rate. 8 frames per second is only really needed,
again, in sport, where the EXACT moment where Andre Agassi hits the
ball and the expression on his face when he does that.
This image displays point 1 and 3. Photographer has to be on focus
tracking while Philipp Kohlschreiber is running for the ball, and
needs the fast frame rate so he doesn't lose the moment. Of course
that don't mean u can just start whacking the shutter when the ball
is coming, you can't do that for every shot, u'll run out of space.
and it's just unpro to be trigger happy.
those are the main three features of pro cameras that i can think
of, and most of us will not use. even in my own line of work, where
i shoot people indoors mainly, i don't need a pro cam.
i'm using a d2hs NOT for any of the above reasons, but becos it's
only a 4mp cam. i'm not gonna explain when i prefer 4mp when most
cameras start at at least 6, if you're that interested you can PM
me to ask.
so to answer your question, it's not whether or not you should get
a pro cam. the first thing is to ask urself, is how rich you are.
buy the best you can afford. if it happens to be a pro cam that you
can afford, then go ahead and buy it, and learn with it.
but the thing about complicated cameras, (which always have an
"idiot proof" setting - ie, full auto for everything) it still
takes a while to learn.
and, as with all hi tech equipment, ur system is as good as your
weakest link. if your body is good, with a sh!t lens, you still
gonna be handicapped. so when u splash out on a pro cam, you also
assumably, will be spending on pro lenses and accessories.
i can't imagine a person shooting sports with a D2Hs, and wanted to
do focus tracking at 8fps with a tamron 70-300/4.5-5.6.... oops...
i think someone i know uses this combination.
So for a "pro" who wants the full range of "pro" equipment, here's
the list (for nikons):
D2x - 8k
10.5/2.8 - 1k
12-24/4DX - 1.5k
17-35/2.8 - 3k
28-70/2.8 - 3k
70-200VR/2.8 - 3k
TOTAL: 18k EXCL memory cards, tripod, flash, batteries, bags,
straps and other accessories.
that would be a minimum, and THAT still does not get the shot i
posted above. I would guess that such a shot would be using at
least 300mm. 
also for your info, shooting in RAW is the preferred quality for
pros, and a d2x produces a 20mb raw file which converts to about
80mb 16bit TIFF file. so yeah u'll need the hardware for all this.
(ask the bear about the quad G5 and it's cost!!). altho, honestly,
not everyone needs to work in 16bit TIFFs..
now the reason why i use "ONLY" 4mp is becoming more apparent.
