mbox access should now be restored, and as Stephen says, now handled by the Date in the message. Let me know if there are any issues with the new ones. (They're now generated by the same code as the per-thread ones).
And just fyi, that also means that complete threads end up in each mbox, so you'll see cases where a message was posted to, eg, -committers initially but the reply cc'd hackers, bringing the thread over there, and now the initial email to -committers and all of the subsequent emails on the thread will be included in the mbox for both -hackers and -committers.
Hopefully everyone finds that to actually provide a better experience, but it's a difference to be aware of.
In theory :), that sounds better for people reading the web archives.
Um. Nothing has changed in the web interface, only the mbox downloads.