Region-highlighting has disappeared.

[Adam Funk]

I just upgraded a computer from Ubuntu Feisty to Gutsy but (as far as
I'm aware) changed nothing else in the configuration; now the region
is no longer highlighted.  (I already have '(transient-mark-mode t) in
the custom-set-variables section of my ~/.emacs, but I can't figure
out what else to do.)  How can I force this back on?

(Strangely, this did not happen with the other Ubuntu machine I
upgraded a month or so ago.)

Thanks,
Adam