It's in Moodle 2.0.
The reason being that it also requires an interface change to allow certain domain and ip ranges to be excluded from the proxy (also in 2.0). It was felt that this was all a bit much for 1.9.
I've closed your report as a duplicate, but I'd certainly appreciate any testing of the 2.0 fix.
EDIT:
Mmm.... something good came out of this. I've spotted a problem with my fixes