I think what BillyBoy is getting at, in an interestingly minimalist way, is that if you visit the Sonic Foundry website, it will pop up a window telling you a little about ACID 4 and offer to let you know when it's available.
I avoid the whole problem by merely bookmarking the entry screen to the forums, proper. I never go through the main SF site. I only see their popups when I go look for updates, 8 packs, etc.
briand, yes, that's a successful method for blocking pop-ups. The problem is that some sites use pop-ups for good purposes. For example, my chat software uses pop-up windows for quick access to quite a few of the functions. In your own case, you missed out on the ACID 4 announcement.