The large aperture (smaller f/number) also gives you a much smaller depth of field or focus area. So it'll blur out the background very nicely. There's so little depth of field at f/1.4 though that it can actually be very difficult to keep anything in focus at all. They have fully variable apertures though, so no worries.
Before you go buying another lens, just park your 28-135 at different focal lengths and see what you prefer. Everybody mentions the 50mm because it's so cheap, but it's also on the short side for portraits. I prefer the 105 to 135mm range myself, at least for tighter head shots. Also, I don't see why you couldn't use the 28-135 for portraits either. It's a perfectly good lens. Just get an external flash for it (430 or 580) which you can bounce off ceilings or from walls on the side and you can take great portrait photos. I do this all the time with my Nikon 18-135 f/3.5-5.6 lens + an SB-600 flash.
