I would believe ISFJ for Sting before ISTJ. I think ISFJ is a strong possibility, with INFJ being my other guess.
In fact, I think calling him an ISTJ is pretty bizarre...he's made a career off of singing mainly love songs, and in his private life has talked about his, um, interesting sex life with his wife and he also has heavy involvement in environmental causes and charities, which might actually point to NF...he's something of a sex symbol, too, in this more classy, sensitive, sophisticated sort of way...since when are ISTJs sex symbols, particularly ones who are revered for their sensitivity??? The reason why I lean more toward ISFJ, though, rather than INFJ is that he does seem more SJ-ish in his demeanor, I do think I see Si more than Ni...but ultimately I think he's gotta be IxFJ.
I just found more than one web site saying that Sting *is* ISTJ!!! He must have a TON of Fi then...