I think I heard recently that it was around 54th in the Nielsen ratings, which isn't great, but it's not terrible either. But with a show like Alias, what it lacks in ratings it more than makes up for in critical acclaim. This show has gotten ENORMOUS critical buzz, for both the show itself and Jennifer Garner, and has been nominated for and won numerous awards (including prestigious ones like Golden Globes and Emmys). That's why it was given the post-Superbowl spot: ABC realizes it has an awesome, well-made show, and they want to try to introduce it to a bigger audience.
Whether that works or not, I'd say it's 99% certain that Alias will be around next season. There's no way they're going to let go of a show with this much buzz without a fight. Also, they need to wait till the fourth season before they can sell syndication rights, which will bring them in a lot of money. I'm not really worried about the show going anywhere anytime soon, I'm just disappointed that the majority of stupid Americans don't know how great it is. But then again, most of them probably aren't bright enough to understand it anyway.