Hah! This is not funny!!! I changed the icon theme (from Human to Humility) in Ubuntu ??.?? (Hey it's a bit of mess here! The sources are maverick, the machine says it's Lucid and the upgrades are multiply botched!) I had Distribution Upgrade running in background (Downloading some new packages) and as soon as I changed the Icons, The distribution Upgrade window vanished.
Just restarted it again. Now it's downloading things. God! I always prefer a fresh install. But I've messed up things so badly here that Dist-Upgrade is the only quick and dirty option I'm left. (Mostly because I'm lazy enough to have a cripple system running rather than doing clean install and installing the hundreds of apps later...)