General Information

Release Date: Spring 2006
Featuring over 50 songs.
Songs are drawn from 5-key and 7-key games.
Game uses the 9th style CS Engine, but looks nothing like it - at last notice, it was quite purple.
Game modes will include (at the very least) single play, double play, battle play, free mode, and practice mode.
There will be unlocks.
Konami will be including an "advanced mode" to entertain existing players.

Known Songlist

The judgement system has changed; there is now a judgement called "Perfect", presumably replacing Just Great. (Source: IGN Screenshot)
Difficulties are listed as "Beginner", "Normal", and "Hyper". Another may or may not exist.
Difficulties are now on a 1-10 scale, using stars. There may or may not be "flashing 10"/10+ rated songs.
Hard has apparently been renamed to "Challenge", or "CHA" for short.
Double and Battle no longer appear as game options, but perhaps exist as separate gameplay modes.
5-key mode is known as "beatmania" mode, 7-key mode is known as "beatmania IIDX" mode. Konami no longer appears to be removing references to IIDX from videos, etc.
In 5-key mode, Free Zones have returned. Free Zones are areas in older songs where you can pretty much scratch as much or as little as you want.

The grading system appears to have changed; it's possible to get a AAA with a money score of 170,000, or with more Greats than Just Greats. (Source: John Stiles)
Songs are divided between 5-key mode and 7-key mode; they do not appear on both. (Source: Bemanistyle)
Difficulties have changed - Nothing Ain't Stoppin' Us is a 5. (Source: Gamespot screen shots.)
The timing window is unchanged. (Source: John Stiles)
Background music is the same as 9th CS. (Source: John Stiles)
The 7-key songs only had their Light 7 charts (labeled as "7 Key"). Pressing/holding Select does nothing. Konami claims this is because they didn't want to scare people off at E3 into not trying the game. (Source: John Stiles and happosai311)
There is no Goli art for result screens in the E3 build. (Source: John Stiles)
There is no doubles play in the E3 build. (Source: sherl0k)
In 5 Key mode, the right side player has buttons 1 and 2 disabled, not 6 and 7. (Source: John Stiles)
The Konami Original Controller is allegedly going to be redesigned to be "better" and at a "more worthwhile price". (Source: djparanoia)
Random+ is on the E3 demo. (Source: chubs)