Update April 2011 - Unfortunately, neither of my hardy agapanthus made it through the first winter. I covered them with bark mulch before the winter, but maybe that wasn't the best way to protect them. Anyway, my zone 5 is a harsh one with very long winters and lots of early and late frosts. These plants might work in other zone 5's or in microclimates. But if you're unsure about how they'll do in your climate, just order one or two to begin with and wait to see if they make it through the winter for you.
Update February 2021 - Other hardy agapanthus cultivars are now available. I planted 'Blue Yonder' two years ago and it came through the winter, though it hasn't grown large enough to bloom yet. It doesn't seem very vigorous in my garden. Last spring I planted 'Galaxy Blue' and 'Galaxy White' and look forward to seeing if they come through this winter. Both increased in size and bloomed for me in their first summer, so they seem vigorous as advertised. I should also note that my area is now considered Zone 6, as we rarely drop below zero in winter.