What is the minimun Focal Length you should use for portrait work? I know 50mm, 85mm, 100mm, are all good for portrait work but is it ok to go any lower? At what focal length does the subject start to bend or distort because of the wider angle?
Well, strictly speaking, the focal length has nothing to do with that. What causes the appearance of distortion is putting the camera too close to the subject or having a steep angle between the axis of the lens and the subject. The basic cause of this distortion is the way rectilinear lenses map their field of view onto the sensor plane. The effects of this are exaggerated the farther away you get from a "standard" field of view, which is roughly that of a 50mm on a full-frame 35mm camera.
I think what you really want to know is, what are the usual focal lengths for portraits such that you can fill the frame with a head and shoulders and have things look right. The answer to this is that it varies according to the size of your camera's sensor.
For a full-frame 35mm sensor, such as in a Canon 1Ds or 5D, the usual range is 80-135mm. The reason for this is that you can get a good facial, head-and-shoulders, or upper-body portrait with these focal lengths at a distance of roughly five to twelve feet. This is far enough away that you don't get the kind of obvious distortion that you would get if you tried to take the same shot with a 24mm lens at a distance of two feet.
For any other sensor, you have to divide the above values by the crop factor of the sensor. For a Canon 7D, 50D, or Rebel, the crop factor is 1.6, so the range becomes 50-85mm.
For full-body standing portraits, the best thing is to stick to the standard range and back up far enough to fit the whole body into the frame. If necessary you can go as wide as 35mm full-frame (about 22mm on 1.6x crop), but that's pushing it. The problem is that the shorter the focal length, the greater the angle of view, therefore greater variation in perspective across the width and height of the image.
As to whether it's "ok" to use any particular focal length, well, that's up to you. If you think shooting close-up portraits with an ultra-wide lens creates a useful artistic effect, more power to you. But it's not what is usually done, and most people would think it just looks weird.
For portrait work, you generally want to use the longest focal length that will let you frame the subject in the space to work with. For the most part, the further you get away from your subject the more flattering it will be.
On an APS-C body, a 28mm or 30mm prime will give decent results for full-body or 3/4 length in a garage-sized or smaller studio. You can go shorter but it requires creativity and attention to detail to pull off well. The 100mm macro is great for head & shoulders or close-up headshots on a cropped body.
If you're working mostly in a studio, I'd suggest using prime lenses. Zooms are nice, and indispensable for location work, but you can't beat the image quality and speed of a prime lens.
Typically in portrait work, we're dealing with head shots and upper body shots. The philosophy behind using longer lenses actually has nothing to do with the focal length of the lens, but the distance to the subject. The distance to your subject alters the perspective distortion of your framing. Perspective distortion results due to the ratio of distances of objects to your frame relative to each other, and the lens, because closer objects look significantly larger than far ones. This ratio is exaggerated as you move closer you are to your subject, and is diminished (normalized) as you move further from your subject. This effect is what photographers refer to as subject "distortion" (not to be confused with lens distortion), and "compression", respectively.
This subject-to-distance relation, while not technically completely synonymous to focal length, can be considered as proportional to focal length in portraiture work, since we generally aim to fill the subject in the frame (there are obviously exceptions). Therefore, the focal length of the lens being used, depicts how far away we stand from our subject. Very few photographers know this, but in psychology, our brain remembers peoples faces with the perspective distortion, as if they were to stand around 10-15 feet away from us, and therefore, matching the perspective distortion from this working distance results in the most "flattering" appearance to us. To utilize this working distance of 10-15 feet, filling the frame with a head and/or upper body shot requires a lens with a focal length of 85-135mm (on 1.6x Canon crop, around 50-85mm). Anywhere in this focal length can be considered as the most "ideal" focal length.
On a side note, a faster lens (f/2.8 or faster) helps throw the background out of focus, drawing the attention to the subject. Faster lenses, such as the Canon 85/1.2, and 135/2.0 are the staple for most portrait photography, although the 70-200/2.8 I and II is also a popular choice for focal length modularity, if the photographer knows how to throw the background out of focus without a faster aperture.
Sorry, Tom, i cannot agree with your comment of: "You don't want wider than a 50mm lens for portrait work on an APS-C sensor camera!" To give a rule of thumb like that would be creatively stifling. Many interesting, often humerus, portraits have been successfully captured using ultra wide angle lenses. Now, if you don't want unflattering distortions, I, personally, wouldn't go wider than a 30mm lens on an APS-C crop camera (50mm Full-Frame equivalent). This focal length will often mimic the perspective our human eye captures. So close one eye and what you see is approximately 50mm. So it makes sense to take portraits at that focal length so people look natural.