the quote system already groups private messages, if you go back and forth with a user via pm all communications are in the most recent pm (grouped). i suppose the only difference would be that your inbox has the conversation history broken up into separate entries. i think a grouped system like gmail would be confusing to inbox size. technically you could have "1 message" (inbox listing) that carries a weight of 50 messages because it is a grouped back and forth pm. while i like gmails style (in some cases i absolutely hate it -- they group emails of a similar title but different message thread, extremely annoying!) i dont think it will translate well into a forum pm system. i voted no.