@hawkeye10,
As Butrflynet pointed out, Gabrielle Gifford's office is doing quite a lot. That doesn't seem incidental or trivial to me.
Gabrielle Giffords was voted in because people agreed with her policies and approach to politics. She hand-selected this group of people who now make up her office, with the intent of working together to implement her vision. In that way, she is still the representative, as these people take her stated vision and priorities and work to make them happen.
If Giffords is unable to recover fully before the next election, I think it would make sense that her second-in-command, or whomever has been most instrumental in the work that her office has been doing since the injury, goes ahead and runs in her place.
I think engineer's point about not encouraging removal from office by gunshot is a good one.