Edit: Well, either I'm mistyping the password
...yep, the username is defined as being 8-bit text excluding a ':' and the password is defined as any valid 8-bit text. The only characters not permitted are non-white-space control codes (0 .. 31). The spec even says to assume ISO-8859-1 for high ASCII.
Right, let's have a look with a packet sniffer...
...ah-hah! Opera transmits accented characters in some sort of multi-byte encoding (probably UTF-8), and there doesn't appear to be a setting to change this. Bad Opera.