I know it's not fair comparing two cameras from completely different price bands but I thought you might find it interesting nonetheless knowing what I think of the K10D after using the D700 for a short time. The short answer is that the Pentax is still a great camera.
In terms of image quality, there isn't a great difference at normal EV levels between files from the D700/28-105 and the K10D with equivalent DA primes. I haven't done any proper testing so I'm not in a position to show comparison pics but output is more or less the same with the caveat that the K10D is at 100 or 200 ISO and the D700 is at 400 ISO. That's the way I have the cameras set up for normal shooting and I'm trying to compare them as I use them. When using the D700 with the 35mm f2 and 85mm f1.8, the results from the Nikon are a little sharper but not hugely so.
Low light shooting is completely different, though. Again, I haven't done any side-by-side comparisons but I'd say the D700 at 6,400 ISO is roughly the same as the K10D at 800 ISO but I prefer the Nikon's output as it seems less splotchy then the K10D's.
These are just some initial findings and the files were jpegs and not raw. If you shoot jpeg and don't do a lot of low light stuff then I can't really see any point in you switching to a full frame Nikon. It's possible that quality differences will show up when I start using raw files and I might have to reconsider some of these findings if I can find the time for back-to-back comparisons. But even if the Nikon shows a slight edge, if you're shooting in good light you're at no real disadvantage with the K10D. I have a suspicion - again, nothing I can prove empirically yet - that the D700 files will be able to withstand quite a bit more post-processing whilst retaining their quality. That will be useful when I start doing some black and white conversions.
There's a big difference in the performance of the K10D and D700 as cameras when it comes to autofocus speed, shutter response, etc. Before I got the Nikon, I never gave the K10D's mirror action a second thought. Now, it seems very lazy in comparison with the D700's. After a few days with the Nikon I picked up the K10D and it felt like I was shooting everything at 1/15th of a second. The D700's mirror action is very rapid and the time taken from the moment of shutter release to mirror return is extremely brief. The K10D's is lethargic by comparison.
Not only is the D700's autofocus faster but it's also quieter and makes a softer-edged noise than the K10D's. It's also more accurate. Even shooting at f1.8 with the 85mm Nikkor I'm constantly surprised at how spot-on the focus is. I wouldn't say the K10D's autofocus is slow - it's on a par with its competitors - but it's just not as fast at the D700's. The Pentax is still capable of focusing much faster than I can manually and in everyday use its speed is just fine.
The D700's exposure meter is very accurate which means I'm doing a lot less chimping and taking far fewer photographs as the first one is usually correctly exposed.
Whilst I love the responsiveness of the D700 - it feels the way good motordriven film cameras feel - the Pentax is by far the more enjoyable camera to lug around if you're out for a few hours. The K10D is a lovely fit for my hand whereas the D700 is a little too big. Even with just the fairly lightweight 35mm f2 fitted, the D700 is still quite a hefty package whereas the K10D with one of the DA Limiteds is just about the perfect size and weight. That's why I'm keen to hang onto the K10D with the Limiteds if possible.
These are two completely different cameras serving different markets and meeting different needs. I can't wait to start doing some serious low light black and white photography with the Nikon. The D700 with the 35mm f2 is a great combination when the light starts to fade or when shooting indoors. Similarly, though, the K10D with the 21mm DA is a great set-up in normal conditions. I'm lucky that I can choose between them.