Wow - I ran Update on synaptic and apt-listbugs tells me that metacity segfaults after restart. I decide to risk it, as it won't stop me booting. The update finishes & I closed it & did Ctrl-Alt-BkSp to restart all the GUI stuff, X, Gnome, etc.
GDM looked slightly different, I logged in & Bam! Big surprise - no desktop. Gnome 3 had arrived.
I love it. There's a learning curve, & probably I need to do some manual cleanup after apt has "converted" my Gnome 2...