In found two mistakes in PEAR/Mail/mimeDecode.php and corrected it :

line 467 :
switch ( $encoding ) {

replaced with :
switch ( strtolower($encoding) ) {

AND

line 278 :
$return->parts[] = $obj->decode(array(
'include_bodies' => $this->_include_bodies));

replaced with :
$return->parts[] = $obj->decode(array(
'include_bodies' => $this->_include_bodies,
'decode_bodies' => $this->_decode_bodies,
'decode_headers' => $this->_decode_headers));

I found the first bug by parsing emails send from Pine witch, in my case, sends uppercapitalized headers.

The second one occurs when decoding a mail with a RFC822 attachment : this attachment wasn't decoded.

Please tell me if my bug report is helpfull for you, i don't know what is behind this reporting bug system.
Thank