This is one of three succulents that Jennifer Schaffer put in when she did our landscaping. They are agave celsii and they are, sadly, monocarpic (die after flowering once).
We're looking forward though to seeing what the flower looks like. It's sort of creepy - I left town for 2 days, nothing; when I pulled up in the driveway, the shoot was suddenly there: at least 3 ft tall.