No, read it again. That's for a representative, not a senator. The governor appoints senators, and the replacement would keep his or her seat at least until the next election opportunity (in this case, 2008).
CNN says Johnson is being examined by the stroke team at GWU hospital.
I don't know. This sentence seems to indicate it does apply to the Senate...
If a vacancy occurs in the office of a senator or representative in the United States Congress...