What is the acceptable current draw on pin #9 on a VGA connector?

It's hard to find out, since you have to pay for the required standard documents from VESA.
It's possible that there is no fixed value, and it's different for different graphics cards to implement as they choose. Since (I think) it's required to power the EEPROM in monitors for PnP, I'd say there may be a minimum value of ~10mA ("covering" all versions).

However, looking around I think up to around 100mA will probably be safe. Here are some links that support this:

http://www.vesa.org/vesa-standards/standards-faq/ - See bottom 3 questions.
Discussion of USB power from VGA port specifically this quote:

I found this from http://www.circuitprotection.com/04Databook/C17_video_(133).pdf :

“Devices that comply with the DDC host system standard typically provide supply voltage on pin #9 of the standard 15-pin VGA connector. The voltage is 5V ±5% and supplies a minimum of 300mA to a maximum of 1A.”

but for DVI:

Per the DVI spec R1.0, the “+5V signal is required in a DVI compliant system… the power pin must be able to supply a miniumum of 55mA and the monitor may not draw more than 50mA.”

To be on the safeside, don’t do this hack on your DVI port.

Maxim demo board that runs from VGA power (using ~10mA)

VESA have made a lot of their standards freely available, so we can finally find out what the standard actually says. The following is from "VESA Enhanced Display Data Channel (E-DDC) Standard Version 1.3" section 7.1.3.1:

All host devices, including battery-powered devices, shall supply +5V ±5% on pin 9 whenever the video port is active. Host devices shall supply a minimum of 50mA, with overcurrent protection to limit the maximum supplied current to 1A (1000mA).

So no guarantee that you can draw more than 50mA.