I suppose it could be that. For me it works on non-piano instruments too. I have a Yamaha PSR-550.
Well, this Creative Labs page suggests that that feature isn't in the SoundFont spec:http://www.creative.com/soundblaster/soundfont/tutorials/welcome.asp?articleid=54142&page=7
There is an envelope parameter called "Sustain" but it refers to the volume at which the note decays to after a while of holding the key. There doesn't appear to be anything in the envelope parameters to behave differently due to the sustain pedal. That's a pity.
Thanks for your help.