I currently have a K100D I am happy with, but that's going to go away in december, and I will be buying a new dslr.
I'm not too uncomfortable switching systems because I don't have very much kit (18-55, 50, 70-300, 1.7x telecon, bag) and the kit can follow the camera, if the entire new system deal is more compelling. In particular, during this switch, I will be looking for a fast normal range lens, because I've realised that carrying around only one lens + camera is much easier than carrying around a camera bag, will make me carry around my camera more often, and having only a 50/1.4 or slow 18-55 is generally pretty limiting (50/1.4 is generally better, but really sucks for many shots).
So - the new camera was going to be a Pentax K10D, and I was going to buy a Pentax DA*16-50 to use right now, but I've delayed that purchase and so changing systems is still an option.
Price is something of a factor, and weight is also a factor. On paper I would pay the price difference for the D300 over the 40D definitely, but there are other possible outcomes I suppose (e.g. if the D300 has unusuable-to-me ISO 3200 while the 40D has usable-to-me ISO 3200, etc.)
The main thing which is holding me back from changing is focal length range availability. The DA* 16-50 would give me 24-75 equivalent. For Canon, there's no way to get to 24mm equivalent in a "standard zoom" because of the larger crop factor and lens selection (without going up to a "full frame", which I will not consider because the cost is pretty prohibitive). For nikon it's close-ish (17-55) but the relevant lens has no IS. I don't think the canon has weathersealing, while the nikon does have it. Given my current habits the weathersealing is probably not such a big deal.
What do you think I should do? The DA* 16-50 is going to be available at COMEX, by all accounts, and I'm so very tempted...
(Note that I see no advantage in switching to Sony/Olympus - their lens lineups are not much better than the pentax lineup, and olympus in particular is unlikely to have new models in time for december.)