Today's SM58 is the same mic', to all intents and purposes, as it has always been. It's interesting to read Stub's post above, which reads as if copied straight from Shure's publicity material. Of particular note is the line "The SM58 is based on the Unidyne III microphone element developed in the late 1950s by Shure engineer Ernie Seeler", which indicates that the SM was a refinement of an earlier model. Nothing in the post speaks of any major change in the design. Sure, components may have been updated, but the fundamental design is exactly the same as it was in the 1960s.
I have a Unisphere from the 1960s, which was the first mic' I ever bought. It's a nice old mic' that has served me faithfully. It still gets an outing occasionally, usually for speech/announcements, when its on/off switch comes in handy. I still use a SM57 for guitar cabs, snare, etc and carry a SM58 because it suits some voices (and because some singers insist on one).
However, I have more modern dynamics - EV n/dyms, AKG D5 and 7, Sennheiser 838 - that easily out-perform the SMs. They have better clarity, more resistance to feedback and a more extended frequency response. Even Shure acknowledges that the 58 and 57 can be improved upon. Why else did they introduce the Beta versions of them?
As I said in my original post, the 58 was great in its day and still has a place now, but it is no longer the best game in town.