You enable it by going to the System Preferences - Keyboard, and ticking 'Show Keyboard & Character Viewers in menu bar'.
I've upgraded to Lion a wee while back, and had not needed the characters palette - but recently I wanted to have a quick look at some Unicode character codes. I brought up the characters palette via the menu bar:
But the resulting palette seemed to be quite spartan and bare and the familiar Unicode table just was not there.
Turns out: the Unicode table is still there, but you need to turn it on first. Click on the settings button, and select 'Customize List...'.
Then scroll all the way down until you see 'Code Tables'. Click the disclosure triangle and tick the checkbox for Unicode.
That adds the Unicode table back. Great for browsing around in Unicode!
Hope this saves someone some time and frustration!