Microsoft never really announced a solid launch date for the update, which caused all this guessing.
Anyway, the WP7.8 update was reportedly RTMed and it should start rolling out in a few days to weeks. This comes soon after the leaked Nokia materials that detailed what changes the update will bring (Microsoft was vague on this one too).
Windows Phone 7.8 will not be the end of it though - another rumor (this one coming from Nokia Germany) suggests there will be another update for the first generation of Windows Phone handsets (the ones not getting WP8).
The rumor doesn’t offer any more info, but that there might be an update at all is quite interesting in itself. Microsoft made a commitment of supporting WP8 devices for at least 18 months after they're released, but it would be nice to see it hasn’t forgotten about the users it attracted with WP7.