I have to disagree with the masses. 150mm isn't too long for portraits. Click on the link below, it's a Shutterbug review of the Sigma 150mm. The first few sentences sums it, I agree 100%. The compression makes a wonderful portrait. I have the Nikon 85mm, 105mm VR, and 135mm DC I use for portraits. And I have to say the 85mm gets used far less than the other two. Compression is a wonderful thing. I have found that a longer working distance keeps clients relaxed. I shoot about 40 weddings a year, and have shot a ton of portraits even with the 80-200mm 2.8 @200mm. I can shoot natural, candid portraits of my couple from a distance that allows them the privacy to be themselves.
I don't believe there's a right or wrong in photography, it's whatever you want. I wouldn't rule the 150mm out, using a long focal length for portraits will set you apart from most shooters. There are quite a few photographers using the Nikon 200mm f2 for portraits, a great piece of glass.