I've been researching the same thing for my 35mm Rebel. A couple of notes..
The 75-300 lenses are big, and their "minimum zoom" is more zoomed in at it's widest setting than the lens that came with your digital rebel when it's all the way zoomed out. This could result in them needing to switch lenses to take pictures of something close up, which can be a pain and a good way to scratch things or introduce dust.
From what I've been reading, anything above about 200-250mm can often get blurry if you don't hold your camera very still or use a tripod, especially if you're taking the pic at slower shutter speeds (Lower light conditions, in general). Canon makes a 75-300 lens that has image stabilization, but it's $400+ new.
Someone mentioned it, but didn't really call it out - Putting a lens on a digital camera results in a LOT more zoom - I thin someone mentioned it - A 300mm lense becomes, to a digital camera, a 450mm lens. Think of that as ~6X the magnification over stock.
Sigma makes a "hyperzoom" series of lenses - They go from 28-200 or 28-300. They are physicallly probably about the same size as your existing lens with all the zoom. No need for messing with changing lenses all the time, depending if you want something close up or far away. The 28-200 would probably be a good choice. It would work out to be something like 45-300mm effective on the digital camera, you wouldn't have to change it out much and it's not too expensive at around $180. Should be fine for the general enthusiast who wants to take some volcano pictures or snapshots of the girl in the grass skirt from across the room.
Mandatory caveat: I'm not a real camera guy, but I've done a lot of the research and this seems to be what I've found works for me. If others with more real-word experience disagree, I'd believe them.
- G