There is no doubt (and practically every reviewer has said as much) that the K10D is a seriously good camera. Pentax at one time was the brand to own and the K10D does an awful lot to put them back in with the big boys. I have always had Pentaxes myself, so am a little biased, but I can say that the K10D is the best camera I have ever owned, and I have had a fair few. It is a camera you are not likely to outgrow - it has just about all the features and functionality of the D200 but at a lot less money. One reviewer called it an equivalent to the D200 but at a D80 price and, in my view, that is a fair assessment. If you do go down that route, you won't regret it.
Having said that, there is an undoubted problem with availability of third party accessories and lenses. Nikon and Canon, being the big boys, have a LOT more independent lenses, flashes, etc, available for them than for the Pentaxes. If you intend building up a whole kit then this might be something you need to consider. Pentax DO make lenses, flashes, battery packs, etc - but you are more limited in choice.
The other thing to consider is the indefinable "X" factor - does it feel good and right in your hands? The best thing is to narrow your search down to a couple and then get them in your hands. Sometimes the "right" camera for you then becomes obvious. I wouldn't downplay this factor - all the big manufacturers make fine cameras and you will get along just fine with whatever brand you buy, but sometimes one just feels more like it's part of you.
Be careful about online purchases .. you don't get to play with them first, and there are a lot of scam artists around. Stick with a tried and tested company - the camera dealer feedback forum will help with that.
I hope this helps, and let us all know what you finally decide.
Cheers
Mike