Im using Kubuntu 8.04 and there is no sort by type yet. It's been over a year looking this topic's title. Found it googling around.
And while you're at it, then I think you should add all possible file characteristics to sorting capabilities. And also, I'd like to customize the columns I see about files. Sure, the name and size are important, but it sure would be nice to see bitrate in case of mp3-s and quality in case of avi-s, don't you think.
You, Dolphin guys, are doing great work and I can only hope you'll add those little features in the not too distant future.