Big Mike said:
Welcome to the forum. I agree with the sentiments of nakedyak, that Canon or Nikon are clear leaders in this field...both in terms of camera technology and lens selection and quality.
However given the choice at hand...I'm with Johnboy, Pentax has plenty of lenses to choose from because the camera will accept any lens with that mount.
I think the Olympus uses what they call a 4/3 mount which is fairly new. Not all that many lenses to choose from.
Ehhhhhhh I am none to sure on that with "that Canon or Nikon are clear leaders in this field...both in terms of camera technology and lens selection and quality" ---Technology? Ehhhh....neither STILL has a anti-dust method nor in camera A/S. Quality? Well, that is pretty wide open as well. I will gladly stack up Minolta G glass anyday against L glass. The reason they are so popular is because they sell more, plain and simple. AOL is still the biggest ISP, doesn't mean they are the best.
Now onwards and upwards to question at hand.
Personally I would probably steer clear from Oly. The reasons are 2 fold for that. The 4/3rds system is expensive and VERY limited to get in to. Remember that when you are purchasing a camera, you are not just buying a cam but an ENTIRE system. There just aren't many lenses available for the 4/3rds...espcially when you compare it to everything else out there. Sony with the Minolta mount, Nikon, Canon, etc. Heck even Sigma's own DSLR's have more of a lens selection! The second reason is straight up image quality. Oly's with the sensors are NOTORIOUS for producing high level noise in anything above 400 iso. That is because the 4/3rds sensor is the size that is in a P&S, where as most dslr's use an APS film sized sensor (unless you go full frame like the Canon 5D)
For the issues I have stated...I would go Pentax. Quite honestly the Pentax cameras are VERY good and with the upcoming K10D, they are going to have a superstar camera if the images it produces out of cam are anything like the IST & K series previously made. Honestly, on paper, there is just not another camera that come close to the K10D in terms of features. The D200 from Nikon is closest, but that camera retails out at $1699- body only. Now considering the K10D is at the very least full 700- cheaper (some reports have put the cost at $799- for a K10D) you do the math.
Pentax lenses, well.....you have nothing to worry about there. You have a MASSIVE array to chose from, and look to
ebay for legacy glass. In fact. from what I have read, even Pentax Manual Focus lenses will bolt right up to the K10D. If that is the case as I have read, you are going to have a camera that probably has the biggest lens selection from the get go so far.
DPreview has a review of the K10D up at
http://www.dpreview.com/articles/pentaxk10d/ I would put this cam at the top of your list, honestly.