The big release for the Source engine was HL2, so...
*crosses fingers again*
The only next gen console that's been announced so far is the Wii U, but it's a given that Microsoft and Sony are working on their successors to the 360 and the PS3.
Valve will never abandon PC gaming. Not with Steam being the juggernaut it is for digital content distribution. And the Source engine was really written for the future, as they've been able to tweak and modify it so much over the years. Take a look at Portal 2. Then go play Half-Life 2. It's hard to imagine they're the same engine, honestly.
Since Half-Life 2's release, Valve has added the following to the Source engine:
High dynamic range rendering
Dynamic shadows
Massive-scale physics (Did you SEE that plane crash in Left 4 Dead)
Whole slew of other things I can't even name right now
Also total OSX support and (coming soon) Linux support for their Source titles.
In fact, it seems that the Linux version of Left 4 Dead 2 is twice as smooth as the PC version.
But it wouldn't surprise me to find out that they're working on Source 2. It'll probably be backwards compatible with all their original Source games, too.
<--- totally a massive fan of Valve's games, so I'm a little bit biased.