Hello Doc:
Welcome to the Pentax SLR forum.
That's a pretty good response for your first ten hours on the blog!
If your background lies in the Minolta camp, you will find the logical operation of the Pentax DSLR system to be very familiar, and the ergonomics are easily the best in the industry.
I will let the other posters describe the many benefits of the Pentax K20D... in this instance, I'll add my two cents from the perspective of a multi-system photographer.
I shoot for magazines, and cover all kinds of events on a regular basis. I shoot with three systems these days, the Hasselblad (film for the real purists), Nikon, and Pentax.
The K20D is an outstanding camera, with very few idiosyncrasies, and excellent metering and auto focus systems. In skilled hands, there is no reason why you can't shoot just about anything with this camera - there are some who say it doesn't focus well in low light, but it does - if you know the technique and become familiar with the equipment. There are some who wish the K20D would focus faster to cover action situations - again, with a bit of practice, and the selection of lenses that tend to focus faster than others, you can work around this - the difference isn't really that substantial between the Pentax and its competition. In many cases it is a matter of technique more than the camera and lens specs - after all, most of the old f@rts posting here took spectacular action images with manual focus systems - it CAN be done, and very effectively.
You do, however, have to pick and choose your lenses after significant research, because there are some beauties, and some that are quite average... as with ANY system, the optics are the critical ingredient.
Check out this thread from the archives for tips on low-light auto focus:
http://forums.dpreview.com/forums/readflat.asp?forum=1036&message=29213895
Now to the heart of the original question.
Do you really need a Nikon to be a professional photographer?
No more than you need hair on your head... it can come in handy at times, but unless you have an obsession for staring at yourself in a mirror, you can't see the name on the camera when you are composing a masterpiece through the viewfinder.
When I shoot Nikon, the images are outstanding, the cameras operate very well, and the enjoyment of the photographic process is quite fulfilling... however, to match or exceed the K20D in terms of results and capabilities, you have to go for a D300 - the D90 is not as pleasurable a camera to shoot with as the K20D, just as the D80 did not measure up to the K10D. The only feature the D90 has that the K20D lacks is video - and personally, when I want video I use a Sony Handycam. Of course, the "burst mode" is a cool feature, but from my perspective it is a rarely-used gimmick.
The Nikon D300 is a pleasure to use, but it is a beast, and in many ways the operator interface is quite a bit more convoluted than the K20D - to the point where it takes much of the "fun" out of photography. I use the Nikon when I need the specific focal lengths that I have in Nikkor optics, but not in Pentax... I use the Pentax when I need the specific focal lengths that I have for the Pentax system. Can I justify owning a 12-24mm Nikon lens when I have a 12-24mm Pentax? Not at all, but I sure wish I had an SMC Pentax 80-200mm f/2.8 FA* lens, even though my Nikkor 80-200mm f/2.8 is a treasure.
Do you need a full frame DSLR (at almost double the cost of the D300)? Not really, unless you are enlarging images to wall mural proportions. The APS-C format is quite sufficient for beautiful images up to and often beyond 16 x 20 inches, which is about the largest you are likely to print from a wedding assignment.
Do you need greater resolution than the 14.6MP in thge K20D? Not really, because it can resolve detail so fine that the skin texture of a beautiful bride could detract from the image - that's also why most pros using the Hasselblad system shoot portraits using a Softar filter - to take the edge off the sharpness of the lens... too much detail can be scary!
In summary, to exceed the performance and enjoyment of the Pentax system, you would have to spend more than double for the Nikon body, and significantly more for each lens.
Is the investment worthwhile? Ask me why I shot weddings with a Hasselblad, and the answer is a blend of image quality, and professional image... walk into a major event with top level equipment and you can demand top dollar for your efforts - assuming, of course, that the photographs match (or exceed) the client's expectations! If not, your last assignment may very well be your... last assignment!
Bottom line: if you enjoy the art of photography, either Pentax or Nikon are wonderful choices... handle the cameras and the lenses, and you will form your own preference - from that point forward, it's only a matter of budget.
Norm.
