So, forgive me but I really didn't read anyone else's posts here in great detail, but thought I would chime in.
I'm a Canon guy myself. I shoot with the 5D Mark I, and have the 85 1.2 L and the 70-200 f/4 IS L, which generally are regarded as the two most popular Canon lenses for portraits. The 85 1.2 for shallow DOF and mid-telephoto zoom for moderate flattening, and the 70-200 f/4 for sharpness and telephoto zoom for extended flattening.
Now, really of all the advice I can provide, is that it really depends on what you want to do with your pictures. I looked at the above examples you provided, but those were all shot with different lenses with different exposures, with different subject framings and different focal lengths.
I think the question you have to ask yourself is what kind of effect are you looking to produce, and what are the tools you'll need to do so. One thing I can recommend to start with, which is moderately priced, is the 85 1.8, or the 70-200 f/4 NON-IS. These two lenses are great to learn to play with your knowledge and understanding of how photography works, and with trial and error can learn about how to create the "bokeh" effects you're looking for.
Generally speaking most people would shy you away from wider angle lenses because they tend to exaggerate the features of an individual. As most have said here however, the wider focal lengths given your camera's crop frame sensor won't really produce as dramatic effects so you're usually on the safer side. I can see why you're likely using the 28 to shoot, because the maximum aperture is higher, and will create a stronger bokeh effect on your subject.
In the meantime however, with your lenses generally the easiest way to capture "standard" portraits is through longer focal lengths. This creates a more dispersed effect of the background when the distance between the subject and background is far - so inevitably the bokeh will be heavier and everything else will be far more out of focus.
But again as I mentioned, it's really up to you with what you want to do with it.
If you like, I can check out my flickr page (vraxtus) for some examples of how I shoot.