Hm... A thought just occured to me... Could it be a problem with the flags? I'm almost certain that I read the Int0 flag was cleared by hardware when the interrupt was edge triggered... But I'm looking through the datasheet now & can't find anything on that. I guess if the flag was still on, it would interrupt every time the int was re-enabled (in other words every time I release the button), making it appear to work in reverse..?
Ach, no, I found it in the datasheet, the flag is cleared when the RETI is executed... Unless maybe SDCC is using a RET? I'll try that, but now I'm thinking that's probably not the issue... Any other thoughts?