If you don't want the K10D, I'd make the upgrade. I almost did myself a few months ago but decided to save my pennies and wait for the K10D to come down a little.
As for why go from the DL to the K100D, the K100D has not only IS, but better software (superior JPGs, identical RAWs since they're not affected by software) and faster focusing (and I believe it focuses better in dark), probably a couple other things, too. DLs go for a decent price on eBay (more than I paid for mine!) so you're probably looking at $100-150 for the upgrade if you sell your old one - not much different than you'd pay going from the K110D to K100D.
Of course, if you don't care about the IS, it's not worth making the switch. But my only gripes with the DL at WDW were slow focusing in dark areas and the lack of IS - add those two and I'd have even more photos that I'd be proud of!
Besides, if you're after a 300mm lens, Pentax is releasing both a 300mm prime and a 70-300mm zoom later this year, both digital-only lenses so they'll be smaller/lighter than existing ones.