Atari decided to get part of its PC catalogue onto Steam. Great news, right? Shame it's for North America only, with no timeline for international users (which includes us).
If you're feeling defiant, you're welcome to try anyway. The links to Atari's games are provided in the announcement post on Steam Powered. However, clicking on them will direct you to the "All Games" page. If, on the other hand, you copy and paste a link and add the US region identifier to the URL, you'll be shown the game's purchase page.
We've dropped Atari's local presence a line to see if this will be opened up to the rest of the world anytime soon.
Update: According to Atari, it's US-only due to licensing agreements, and unrelated to the current state of the dollar.