Actually it *does* resume, but the progress bar does not. In other words, if you download half, disconnect and then reconnect, the download will begin at the half way mark, but the progress bar will begin at 0% (so when you have downloaded 75%, the progress bar will show 50%).
Though it may be annoying to have to redownload game content, the resume on the downloads, coupled with the generally good transfer speed from Steam servers (apart from the gridlock that occured at launch) make it as painless as possible. Also- remember that Half-Life is the last game you will have to download. You will be able to install HL2 and directly integrate into Steam. No big download necessary.