Does it work correctly for you?
Yes, using FF 1.5 and PasswordMaker 1.4.4 beta1. MPW stored in memory. No custom accounts... I just checked auto-populate for the defaults and typed in any username. Is this what you are doing?
Ok, tried from my home computer, same thing...
I did notice one thing though, that I'm maybe doing differently...
If I simply hit Alt-` again, the password gets populated immediately...
But if I close the page (I still have FireFox running, with the Forums loaded), then click the link you provided again, it again prompts me for the MPW, but it does still say 'In Memory'.
Were you closing the page?