possibly a female Libellula Plathemis lydia (Common Whitetail Skimmer), or Libellula pulchella (Twelve-spotted Skimmer), but i’m not sure…

ETA 100611: according to an entomologist who specialises in odonata, it’s actually a Libellula pulchella (twelve-spotted skimmer). here is another photo of pulchella, which doesn’t have the white spots on the wings, and does have yellow spots on the back of the eyes, though, so i’m still not completely convinced.