this is a primitive, so it belongs in the
pdfTeX manual and not in anything from LaTeX.
We're still talking about \pdfstrcmp? Paul Isambert, Thanh, Hartmut,
.., and I recently (Nov'10) released an update to the pdfTeX manual
which includes that, among other "missing" primitives which had been
implemented for years but never inserted into the doc.
http://mirror.ctan.org/systems/pdftex/manual/
Best,
Karl