In what genre of music would you place the band ELO (the Electric Light Orchestra)?

Although they were masterminded originally by Roy Wood, who sadly left them after their first album, I wouldn't call them Glam. In fact under RW I'd say they were the perfect fusion of Rock, Pop and Orchestral music.

Although the spectre of Prog Rock might rear it's ugly head occassionally, they could never be called Progressive under Jeff Lynne. In fact I'd go so far as to classify them under Pop rather than Rock.

Orchestral rock.

Or orchestral maneouvres in the dark.

Orchestral Rock seems like as good an answer as your ever going to get, though you might say Orchestral Pop as not all they did might be classed as Rock.
Jeff Lynne's influence can be heard in all of his subsequent production work, whether Travelling Wilburys or George Harrison, Roy Orbison etc...he managed to stamp his instantly recognisable sound.. A bit like the Phil Spector Wall of Sound (in that you know straight away when you are listening to his productions)
I wish I could come up with a better description, but I can't better 'Orchestral Rock'