Pixiq

I used to shoot with a prime 105mm lens on my Nikon F3 as a portrait lens. I'm not sure that there are any lenses other than a tilt-shift lens that would let you defocus while shooting, but there are ways around that. You could use a wide-open f/2.8 or faster aperture. Or you could use software to blur the background of an image of a portrait subject. I've used Bokeh in the past and it worked really well. It is from Alien Skin. There is also a program called Focal Point by onOne Software that is supposed to do similar effects.
I would ask some of the Pixiq Experts to weigh in but I think that you'd want to shoot sharp in camera with a wide aperture or soften the photo afterwards in post-production rather than shoot soft-focus to begin with. I think part of the reason that we used soft focus filters so often with film was that you weren't going to get such consistency if you softened the image in the darkroom while printing, but its a different ballgame with digital.
Hope this helps.

The 105 is a great focal length, and Nikon has a fantastic 105 f/2.8 Micro. The thing is, because it's a crop sensor, you're effectively getting a 152.5mm, which is quite a long focal length for Portraiture (but by no means too long, as the 200mm f/2 is a very important lens for many portrait photographers). You may find it a bit long though, so I would suggest one of the 35, 50, or 85mm fast primes (f/1.4 if you can afford it). That will hopefully give you the soft look you desire wide open. As Diane says, you'd have to use either a LensBaby or a tilt shift lens to get a soft focus - an option you may wish to consider.
I would prefer to get the shot as I want it in camera, and therefore opt for a tilt-shift if I were you, but I am a purist and therefore a bit biased.
Oh, and I just remembered - you could perhaps search for funky filters for the front of your primes - there are a wide array of them, and some of them might deliver the results you're after.

Best,
Mark

Personally I'd look at a fast 85mm. This'll work out at around 135mm on the DX sensor, which was traditionally a great lens for face shots. Or for something a little wider, try a fast 50mm. I seem to remember that a while ago, some nikkor lenses had a soft focus function built in but I don't know if this is still the case. If not it might be worthwhile looking for an decent 2nd hand manual lens with this facility. As long as it's an AIS lens, its fully compatible with any of the D300 range.

The budget option would be a 50mm f1.8 (75mm equivalent) which is not a bad focal length for portraits. f1.8 is fast enough to diferentiate subject from background, and is also a usefull low light optic. (won't autofocus on the entry level cameras though). For portraits its no bad thing to focus manually though.
Next up would be the Tamrom SP90, which is a nice macro lens but also well regarded as a portrait lens. Its an f2.8 optic still fast enough though for most situations. Again this won't autofocus on entry level models.
Then there are the 85mm Nikors which are available with AF-S (autofocus with all current cameras) The f1.4 has excellent bokeh, but costs a bomb. The f1.8 is a fine portrait lens but if you want the best bokeh the f1.4 is the king of the hill (if you can afford it).
You can still get some great results with the 50mm though.

prime lens... 35mm f1.8, 50mm f1.8 or 85mm f1.8. if you have a deep pocket then you can go for the f1.4's

theses lens are perfect for ambient light and even low light shooting.

105 mm lens photography portrait is good.

http://www.wholesaleonelectronics.com/cameras/