I am finding the 31 to be a gorgeous lens on the istD. It could be either a bit smaller for it's speed, or a bit faster for it's size. The 58mm filter size is a pain, what with the non removable hood requiring that filter size be used. The only other lens I have that takes 58mm is the 100mm macro, and I haven't bought many filter for it, since it doesn't get out much.
Have you tried a step-*down* ring? You might be able to get to 55mm without vignetting on the *istD... just a random thought :-)
S
