Supergirl is back in the 21st century and is now a member of the Teen TItans. She really was the current DCU Kara who got propelled to the future at the end of one of the Infinite Crisis tie-ins. She was returned by Brainiac 5 with a her memory of the time she spent with the Legion wiped out. This occurred a couple months ago in the LSH book. Her return corresponded with WWIII in the present day DCU. She got her memory of the time she spent in the future back in an issue of her own magazine a few months ago. That was an issue that Karate Kid and Una (the former Triplicate Girl) appeared in. It wasn't the Karate Kid and Triplicate Girl of the Legion that currently appears in LSH, but the ones from the Lightning Saga Legion who have been featured in Countdown.
I think that is right... or at least close. It was kind of confusing because it was presented all out of order.
The Polar Boy in the 21st Century is also from the Lightning Saga Legion, and is currently appearing in Action Comics. This version of the Legion was promoted as the return of the pre-crisis Legion which, of course, we have debated to the Nth degree here. (See the Lightning Saga in Action thread).
In the ongoing LSH book, Cosmic Boy is gone, mysteriously taken to the future by a three teen age super heroes. Mon-El is back in the Phantom Zone. Lightning Lad is the new leader and Jim Shooter is back writing the Legion for a 16 issue run with new artist Francis Manapul. They are introducing new characters, giving a lot of members new costumes, and taking the book in a new direction.
Caught up? That's about the best I can do for now.