I can't exlude for sure that there are differences between the 57 and 58 apart from the grill. However, a grill can make a big difference (read my earlier quote from Shure), and also the placement of the grill (e.g., how far it is from the capsule and how close you can mike).
If there are differences in the design of the amplifier (according to Shure, the capsules are identical), they must be small, though.
So, if the 57 and 58 are almost identical, why would Shure not clearly say so? Well, first off, its a matter of economics: they develop one mic, change the head a bit, and voila: they have two products to sell! Combined with a good marketing strategy (one mic is for instruments, another for vocals), they are very effective at selling basically the same products in two different versions.
So, hey: I'm not saying that I know for sure that they are the same mics, but given that the design is almost identical, that even Shure says that they are based on the same design, the interchangable grills, and from what experienced recording engineers told me, they seem to be near-identical in my opinion.
In any case, my point was: they are both one of the finest dynamic mics ever built and it really doesn't matter which one you get