I hope they remain private. Just imagine what would happen to Steam if Microsoft bought them, or wait, I don't have to, we already have GFWL (barf). I had to create an XBox Live! account just to play Batman: Arkham Asylum, and I don't even own a damn Xbox. Oh, and I had to manually install an updated GFWL client just to get Kane and Lynch to start, cause the GFWL client that came with the game didn't work. No, Steam works just fine as-is.
As for Valve and their game development, they aren't releasing games any slower than Blizzard, and Blizzzard had Vivendi/Universal, then Activision "backing them up". I think smaller companies get better work done, cause some corporate suit isn't breathing down their neck all the time with crap about why their productivity metric is down and how they are not capitalizing on the growth rate expenditures of their target market, thinking outside the box, brainstorm, SAP, visualization, Web 2.0, meaningless catch word, blah blah blah. You know, the same crap upper management always spews out.
Until Valve says: "HL3 isn't coming out because we're out of money", I don't know why they'd even consider selling.