It looks like Lucasart's success with their own adventure games on Steam woke up Activision Blizzard (the current owners of Sierra properties). You can now purchase both Space Quest Collection and King's Quest Collection on Steam. Oh, and they are 25% off until July 30th. On the downside, they seem to be unavailable to customers in Australia.
However, before you jump to Steam and buy them, do note that unlike Lucasarts with their proper Windows-native ports of old DOS games, Activision couldn't be bothered to do anything to polish up these releases. They are bit-for-bit same as old boxed versions of the same SQ and KQ collections that were released over three years ago. That means you may have issues with modern 64-bit operating systems and you have to run the DOS games using DOSBox.
People have reported that while all Space Quest games (Space Quest 1-6) do work on Vista 64bit, King's Quest 7 won't run on 64bit systems. You will also have to dig up the included PDF manual as the old "look up from the manual" copy protections are intact. One must wonder how much better these packages would've sold if Activision had bothered to recompile them all as proper Windows applications. Oh, and get us Quest for Glory series and all the other great Sierra adventures that are not included in these collections.