I don't know the name of this butterfly. - (Sep/04/2012 )
Probably not a butterfly, rather likely to be a moth I suspect. Don't know the name however. It would help if you posted your location (country and region) so that it could be narrowed down.
Someone says it's white-lined sphinx moth
Google imag search tells me that is a possibility, but I think that isn't quite right, the vertical lines on the wings aren't very pronounced and I don't see the underwings in the google images...
Definitely a hawk moth. Likely a Hyles sp. Perhaps a Hyles lineata. Depends how much colour variation in this species occurs (and it has to be a lot then, as wings, tegulae and thorax look quite different). And of course also where it comes from as bob1 mentioned, i.e. if the white-lined sphinx appears there too.
It's definitely an Impatiens hawk moth:">
Thanks bob1 and hobglobin
Images I found for the Impatiens hawk moth definitely look more like the picture you posted.