The only disadvantage that I can see about the K10D is that it will be a bit bigger and heavier that K100D and K110D, but it's not dissimilar from its competition. There are demo photos on Pentax's Japanese web site which are stunning - I'm hoping that independent reviews also get good image quality results, that's the only real question mark at this point.
18-250mm sounds like an awful stretch for a single lens. I think you'd be better off with two different lenses.
Handicap18, I don't think we'll see in-body IS from Canon/Nikon for another cycle or two - I just think that it has got to happen, sooner or later. I'd expect that in 3-4 years, virtually all digicams, from 3x zoom PnS to top-end DSLRs, will have sensor-mounted IS.
Feel free to call me out later if I'm wrong.

It's just my guess, but I think I'll be right. Certainly, if one or the other does, the other will have to follow without a generation of two of their own DSLR.