Minor change request:
Any OPF file created by Mobi Creator seems to start with the 3 byte sequence:
EF BB BF
immediately before the opening tag <?xml ...
These three bytes make "opf2mobi" give a "junk before XML element" error. Would it be possible to make the tool ignore them?
A little research shows that this is a Unicode "Byte Order Mark" sequence indicating that the file has UTF-8 encoding. See:
http://en.wikipedia.org/wiki/Byte-order_mark
for details.