Vim - surround comment line with comment character
11 8 7 keystrokes

YpVkr#p

Yp - duplicate current line, leaving the cursor at the lower of the two
V - enter visual line mode
k - go up and select both lines
r# - replace every selected character with #. Leaves visual mode and leaves cursor at the upper line.
p - put the yanked line (the original) on the next line.


(thanks to doorknob for reminding Y=yy)