There are a number of lenses that can "double" as portrait and macro, as far as focal length is concerned.
Among macro lenses lot of people like the Canon 100/2.8 for both. Personally I think it's a little long for a lot of portraiture situations when used on any of the 1.6X crop sensor cameras.
Some alternatives are the Canon 50/2.5 (which needs an adapter to go to 1:1 macro) and EF-S 60/2.8, Tamron 90/2.8, Sigma 50/2.8 and 70/2.8. Also there's a Sigma 105.2.8, but like the Canon 100/2.8 this may be starting to get a little too long for some portrait work when used on a 40D.
Focal length aside, there are still some reasons you might want to consider separate lenses.
For one, macro lenses can be literally too sharp for some portraits. You may upset some of your subjects with the images you make with one! Not everyone wants to see their every pore, nose hair and blemish.
Also, macro lenses are optimized for their smaller apertures. Not that some don''t work very well wide open. But, the biggest challenge in macro shooting is getting adequate depth of field, and to deal with that you stop the lens down, so that's how these lenses are designed to perform best. In fact, macro lenses often have f32 or even f45 apertures, smaller than most non-macro lenses.
Another factor, macro lenses have to have a lot of focus travel, either internally or externally, to manage to cover from infinity to 1:1 magnification. This makes them, on the whole, slower focusing. There are some with features to help with this. The Canon 100/2.8, for example, has a focus limiter switch, that helps a lot. It also has USM, which is faster than a non-USM lens (Sigma offers some HSM lenses, which are similar. Other lenses with out USM or HSM may be slower focusing.)
Slower focusing in macro isn't a problem, because much of it tends to be done with manual focusing techniques, anyway.
And, on the other hand, there are "traditional" portrait lenses. They usually have a large max aperture, f2, f1.8, f1.4 and even f1.2. Plus, they are designed to work closer to those max apertures, and are not so optimized for their smallest apertures. They won't have the very smallest apertures a macro lens will. Most only stop down to f22. A few might even stop at f16.
Here you might consider the Canon 50/1.8, 50/1.4, 85/1.8 and perhaps even the 100/2 lenses. Sigma has just introduced a 50/1.4 non macro lens, too. Again, the 100mm may be a little long for some portrait work on a crop sensor camera, but the f2 aperture sure is nice.
Now, with any of these you can add macro extension tubes to the back of them, to do some macro shooting. If you plan to mostly do portraits, occasionally do macro shots, this might be the better option.
If you mostly want to shoot macro, only occasionally need to shoot a portrait (and know how to soften a too-sharp image in Photoshop), a macro lens might be your best choice.
But, you can see from the above explanations why some people have both macro and portrait lenses, even though the focal lengths may be very similar.
