I have found that with a steady hand I can get a reasonable photo of an aircraft flying at what I assume is cruse altitude using my Canon EOS 500d and 55-250mm lense
http://www.flickr.com/photos/45701084@N08/sets/72157624293800574/ see photos here,
Also Raymod has made up an adapter so he can take his 55-250mm lense out to 500mm, hopefully he can link his photos here soon.
The sky is clear tonight so hopefully spotting will be good tomorow morning.