##POSIX sh + GNU find + GNU mv 67 ASCII bytes + one (literal) space
find -d -exec sh -cf 'IFS=\ ;IFS=_ set $0;mv --b=t "$0" "$*"' {} \;
I don't know if it fits, but with this any sequence of spaces is elided to a single _ - I like it anyway. It just uses the internal field separator to separate fields.
It's fairly... chatty...
...oh man. I knew the tab thing was cheap, but I thought it was at least clever. Now I'm just late to the party...