The good news: the Orange Box particle editor is out, along with accompanying material and commentary editor.
The bad news: they're so buggy you can't even save your work without crashing the engine.
Oh well. They're worth exploring regardless: to access them, launch any Orange Box game or mod with the -tools parameter. Note that that will only work if you launch through Steam - the param will crash the engine immediately if you're launching direct from hl2.exe.
(You'll have to manually extract .PCF files from an existing game's GCF if you want demonstrations of the new particle effects.)
Good on Valve for releasing yet another buggy update!