Even though the law of the day put all the power in Ananias’ hands and even though, according to Peter, Satan had put it in Ananias’ heart (not necessarily Sapphira’s) to lie about the sale of the property, because Sapphira knew about the deal and, like a good wife, decided to go along with him, Peter said that she had “agreed together” with her Satan-inspired husband to test the Spirit of the Lord.
The reason Sapphira died was because she did not cherish her union with Christ as more precious than her union with her man. While it is certainly stated in Scripture that a Christian women must submit to her husband, this is only done in the context of a more enduring and important reality. While it is true (and even mysterious) that she is one with her husband, that wonderful union is merely an illustration of a better union.
Thanks to my friend Wendy over at Practical Theology for Women here's a link to something interesting I'd encourage you to go read (and her thoughts on the blog entry, too).