From PSO-World.com:

Viewers of Sega's live video stream
from the Tokyo Game Show were the first to catch a glimpse of Sega's
upcoming Phantasy Star game for 2011, Phantasy Star Online 2! The
surprising reveal took place at the end of a montage which showcased
every Phantasy Star game from the past 10 years and also included a peak
at Phantasy Star Portable 2: Infinity, which was recently revealed
as well. While no actual game footage was shown, a logo for Phantasy
Star Online 2 did appear at the end of the video, along with some basic
information indicating that the game would be released sometime in 2011
(presumably in Japan) for the PC.

It looks like this is indeed PSO's 10th Anniversary surprise that many
were hoping for and we'll likely be finding out more once Sega releases
an official statement. We're obviously very excited about this news and
will post further details about PSO2 as we get them. In the meantime,
check out the the full video HERE (Posted at SegaBits) and feel free to drop by the PSO-World Forums where an active discussion is already taking place about this awesome news!

And also

Resident translator Espiokaos has been kind enough to post a lengthy list of new Phantasy Star Online 2 details, taken from last night's TGS teaser video.
While some of the details were obscured in translation, there are some
very interesting bits that should keep the speculations and discussions
going strong until we get some more concrete details about this
long-awaited sequel. Here is the list of translated details (some of
the more interesting or significant changes are in bold):

"Action" and "RPG" re-connected

Increased variety of client orders

Map functions changing in real time

User-created story

New interface for environmental interactions

Global support

Thorough racial balances

Full user participation events

Seasonal changes

New chat system

New mail functions

Drastic item synthesis overhaul

Even wider variety of ways to raise your character

New direction of event scenes

Approach from NPCs (possibly a new approach to NPCs?)

Silhouette changes by add-ons (obscured)

New photon arts (obscured)

Community supported content (obscured)

Additional character customization

Category-based weapon training system (obscured)

Implementation of asynchronous communication

New party system

Improved enemy AI / cooperation

Limitless content

New approach to the game client

Implementation of a new "life work" (obscured)

Daily surprises

"Trump card" system

Mutual interaction functionality for new and veteran users

There were also a number of words and phrases used in the trailer
which may give some insight about PSO2's story and setting: "Awakening",
"Sealed stone monument", "Pure Dark Falz" (obscured), "Apostles of
_____" (obscured), "Camp ship", "Arkz", "Why is darkness born?", "Why is
the Great Light no longer here?", and "A world in ruin." Talk about
one heck of puzzle to put together! The full, original list can be
found here.

Feel free to post your predictions or interpretations in our comments section or Phantasy Star Online forum! Thanks again to Espiokaos and Shougai PSO for putting together this great information!

As a huge fan of the Phantasy Star mythos in general and PSO and its successors in particular, I am massively excited about this! I can't wait to see the actual game, and I am hoping like heck they bring it to the Western audiences!

Anyone else excited?