There is such a huge difference in the noise from the 550D at different temps. My sensor generally sits at about 10 degrees above ambient during an imaging run - so the last few nights it has been 25-26 degrees - but during summer is more like 37-38 degrees. The noise is much easier to deal with at the lower temp.
I will dig up some darks and post them.
That is a cracker of an image by the way
!
Adam