There isn't a specific soft focus or diffusion filter that ships with PPro, but you can roll your own effect, and even save an effect preset that combines all the necessary filters to make it easy to reapply later on. I did just that, and uploaded the preset in a Zip file; grab it right here. Import the preset, and apply it, and you'll see how it works.
The Brightness and Contrast filter is off to start with; tweak the other parameters and then turn it on and adjust it if you need to correct the exposure of the composited final shot.
The Fast Blur applies the diffusion; you can adjust its value to taste.
The Compound Arithmetic filter is what does the magic. What it does is composite an original copy of the clip in Screen mode onto the blurred clip (and potential the B&C filter, if you use it). You can also try Add mode or Copy; I just happen to like the Screen look. Adjust the "Blend with Original" parameter to change the amount of the diffusion.
If you tweak it to a look you like, or something that works better with your footage, Ctrl+click the effects and select "Save Preset" from the flyout menu. That way, you can easily reapply it to other clips in your project, or in a later project.
Hope that helps!
Whether it meets the OP's needs, that is a great technique and many thanks for sharing it.
Download the latest MB Looks version it works with vista 64 now. Came out a month ago.
I second Josh's recomendation for Magic Bullet Looks. It ain't cheap, but I seriously question the professionalism of anyone who produces in Premiere without it. This is one must have plug-in.