Interesting. It looks okay for a frontal crash:

http://www.iihs.org/ratings/rating.aspx?id=222

They're not so fond of the side impact. On a scale of Good/Acceptable/Marginal/Poor, they rate the Ion Acceptable for a frontal crash and Poor for a side impact (even with the front and rear head curtain airbags).

http://www.iihs.org/ratings/ratingsbyseries.aspx?id=526