This worked perfectly when I was using Logic Gold 6 & running Panther. I upgraded to Tiger & Logic Pro 7 at the same time which then required re-entering my registration code due to the different configuration Logic Pro 7 uses compared to earlier versions. Since then SoundFont Synth no longer works as well as it used to. I will open & start playing a song but the track that contains SoundFont Synth will be silent until I play a few midi notes into it which seems to "unlock" the plug-in. However if I want to bounce the song down I'll find the SoundFont Synth track is silent again.
I should add that some other AU plug-ins that used in Logic 6 do not work at all in 7.