Are you talking film or digital, because what is suitable for portrait will differ?
On film the M choises would be M85/2 (the lens in jsherman999's thread), M100/2.8 and M135/3.5 (well, there is an M120 also, but I've never seen in other than on picture). I have the younger brother of the M100/2.8 (the A100/2.8 with identical optics byt the A contacts), which was my first Pentax tele. Good alround short tele. Bashed by people saying that the K105/2.8 is better optically, but the M/A100/2.8 is more compact. Same goes with the M135/3.5, not the best Pentax 135, but far from the worst and smalish. A lot of quality per pound. I'd love to have the Pentax M85/2. It is usually listed as the worst Pentax 85, but it is only because the others (f1.8 and 1.4 are sooo goood). But the M85/2 is really compact (smaller and lighter than the FA77 ltd). If it was made today it would be called a M85/2 ltd for sure. It would fit nicely in my pocket. It's on my shopping list.
If you shoot digital all of a suddenly the 50's are worth considering (and the M135 may be too long) for portraits. The M50/2, M50/1.7, M50/1.4. Don't know about the f2, but the f1.7 and f1.4 are excellent normal lenses that now work fine as short portrait lenses on digital. The f1.7 does not cost much and is realy a lot of quality for the money.
I'll see if I can dig up some portraits, but I tend to use the FA50/1.4, A*85, A*135 or DA*50-135 for portraits. The M lenses still get used when I get in the mode for film and street shooting. Oh yes, I have some portraits with the A100/2.8 on flickr (same as the M100/2.8) and A50/1.7 (same optics as the M50/1.7).
A50/1.7 at f2.8 on the K20D:
A100/2.8 at f2.8
If you don't exclude lenses from the K generation, consider also the 55/1.8 and 55/2.0 (basically same lens down to the last gram but in budget version). They go for almost nothing, and are a little bit softer than the 50mm lenses, with a warm tone that suits portraits well (on digital).