Is there anything I can subscribe to that will let me know when expansions have been released for purchase?

No, but you'll hear about it first on boardgamegeek.com.

You can, actually. If you go to the main site for Netrunner, there's a link for an RSS feed for the news articles on the right hand side (just beneath the list of news items). Every expansion release is usually accompanied by a news article.

Thanks, I'll check both of those out.

Edit: NM