On a fresh install of essex trunk after an image has been launched by some means other than the euca tools the first time that you run euca-describe-instances it fails with
ImageNotFound: Image 51962e9a-7e34-4bec-b73b-51bdd0be963b could not be found.
If you then run euca-describe-images then you can run euca-describe-instances without error
RESERVATION r-1w0xipjb 78f135077acc47a5aed81218401fba24 default
INSTANCE i-00000001 ami-00000002 test1 test1 running 0 m1.tiny 2012-03-06T15:42:38Z nova aki-00000001 ari-00000003