Unless you've been asleep for the past 6-12 months, this isn't new information. Obviously, many here have been ignoring the posts about these details regarding the SP game or didn't bother to go the Steam web site to find out the information for themselves. If they had, they would have learned the very same information months ago. You must connect to Steam to validate your CDkey (or whatever) the first time you install and each time you reformat.
As for me, I'm not buying HL2 until I see some reviews. I got burned buying Doom 3 when it first came out and I'm still forcing myself to finish. I'll wait to see what the forums are saying and what rating Computer Games Magazine gives it (they gave D3 3.5/5 stars, which I think is pretty fair). If HL2 looks good, I'll purchase and then download the inevitable hack so I don't have to install Steam as I have no interest in MP.
One thing I'm still trying to find from those who use Steam is whether your download directory (the directory with the dot-qcf files) is the same as your install directory. In other words, do you effectively have two complete copies of every game on your system (i.e. double the hard drive space, which would be yet another reason not to use Steam)?