This was my question, so I looked at the source. It refers to a page on algorithms that claims that it is 8-bit clean, i.e., it preserves non-ascii characters but does not modify them. I then compiled a test program to verify this, because the bit twiddling in the source code looks a little nuanced. It IS 8-bit clean.

Although, if you're looking for something that converts non-ascii characters to their lower case equivalents, just use LibICU. This library looks nice for decoding things like HTTP which have plenty of ascii-only case insensitivity.