The problem is common.mpq is a new type of archive that is an extension of the old MPQ format. This is because it became so big it would no longer fit (the space allocated for the hash table is too small, I believe, but that's not important... you just need to know the format changed slightly to accommodate this).
I know the new format; there are several white papers for it on the web; and I've been spending some of my off time writing up an extractor for this new format, but it's no easy task, of course. If I ever do manage to complete it, I'll let you know, though. Until then, I'm afraid to say I haven't found an alternative method to extracting it. Perhaps Blizzard will release a new version of the extractor for us in the future