From somewhere inside Microsoft, musings about a possible future interface in Outlook:

Conversation Clues
In a conversation where a message gets multiple replies, a simple chronological view of the messages isn't enough to convey the relationships between the replies. For example, if a message gets two replies, and each of those two got a single reply, your conversation would now have branches. Grand Central handles these relationships by showing lines along the left edges of the messages. You can see at a glance how many branches a conversation has. To help you follow just one person's views - say your boss or the company guru -- Grand Central uses colored lines to connect replies from the same person.

[ComputerZen]

Perhaps this could also end the “quote the entire conversation so far” emails that we all seem to get. And, conversely, end the “what was your terse response in response to” issue as well.