They use the same plasma panel (and properly setup the 550 is not "more dim" per the other poster). The differences are:
-- The 860 has a much better anti-glare filter. This will not only help with reflection but increase perceived contrast.
-- The 860 has Cinema Smooth for 96Hz playback of 1080p/24Hz content (the 550 will play it at 60Hz using 3:2 pulldown).
-- The 860 has the Internet and home networking features and plays more media types via USB than the 550.
-- The 860 has a few more picture control options in the user menu and more importantly has a full CMS for professional calibration control of gamma, etc. in the service menu.
-- The 860 has only 1 Component/Composite input -- the 550 has 3.
-- The 860 has the worst internal speakers ever on a large TV.
-- The 860 is half as thick (1.4" versus 2.9").
-- The 860 is not available in 63", the 550 is.
In the new models coming out within a couple months, there will be 63" versions in the super thin, full featured models.