I purchased my Nikon D300s for a simple reason: I was sold on the D300 and I’m a stickler for getting the current model. I’ve been very satisfied with it.
Now Canon has come out with the 7D which lines up directly against the D300s. DPReview has a really good in-depth write-up of the Canon 7D. Look at this page for a comparison of the 7D, D300s, and the Pentax K-7. No surprises there, but what I found interesting is that the K-7 is much cheaper and yet competes with the two big boys.
I’ve invested in Nikon and unless they suddenly go out of business, I’m not getting a new DSLR system. But for new buyers Pentax might be a workable option.