While we wait until this April 28th for Valve's official announcement on the Half-Life sequel, a few details have been leaked out already as well as some screenshots (mostly magazine scans). I took the liberty of copying what one of our forum members posted in this HL 2 thread in the forums, thanks Svoboda...

These are the details known so far, can't wait for those high-res shots...
» Half-Life 2 is scheduled for a September 30th release.
» Yes, you're still Gordon Freeman, and yes, the story assumes you went to work for the G-Man.
» Half-Life 2 takes place in a European city called "City 17."
» Headcrabs will be back! Although some of the Xen aliens will be your friends?
» Barney is back. And so is Half-Life's original writer, Marc Laidlaw.
» While the new engine has all sorts of fancy features, it's still designed to work on lower-end machines. Apparently a 700mhz processor and a video card capable of running DX6 is enough, although a 2ghz with a GeForce4 is recommended.
» The girl in the promo art is named Alyx, a daughter of a scientist who factors into the story somehow. She's not playable.
» Scripted Sequences will be more interactive.
» Half-Life 3 is planned, so expect another cliffhanger ending.