Digital Rebel & Sunpak Flash Issues

I just got a Canon Digital Rebel. Previously I had a Canon EOS Elan
film SLR, and the flash I've used with it is a Sunpak Power Zoom 4000AF
which has always worked fine with my Elan, and worked fine just this
minute when I tried it.

However, when I attach it to my Digital Rebel, it doesn't seem to work -
the flash seems to identify the lens focal length (it changes as I
change the zoom on the lens), but when I look to take my picture, the
Rebel beeps and blinks the lightning-bolt icon as if to say "I need a
flash!" but is not otherwise recognizing the external flash or using it
- it just takes the photo (or wants to) as if the flash wasn't even there.

Is the Rebel simply not compatible with certain types of external flash
units? Can someone fill me in on what might be going on?