I'm not on gmail, so I don't know if this is the same as conversation-style, but Thunderbird has provided the ability to view emails in a threaded view for years. Apparently you can also configure Tbird to maintain this threaded view when sorting by date, subject, from, to, etc.:
Tbird's threaded view is nowhere near as intelligent as Google's threaded view. When an email client arbitrarily adds a "Re:" is messes it up. Conversation-style view is more or less useless when split up among multiple conversations.
When an email client arbitrarily adds a "Re:" it messes it up.
I'm pretty sure Thunderbird uses the Message-ID and In-Reply-To headers to create the threads, not the subjects. But some people don't always use the Reply function, or Reply to an old email when they want to start a new conversation. Maybe Gmail has some logic to get around these limitations.
In Thunderbird 3, we use a combination of the two models, as some mail providers (Yahoo! for example, IIRC) strip away the right headers. Thunderbird 3's threading is way better than Thunderbird 2's.
http://lifehacker.com/264317/maintain-threaded-view-in-thund...