Generally speaking webfonts in all their incarnations are disastrously prone to FOUC, and there's little you can do about that... The nature of the beast.
My question though is "good lord why are you still using Cufon?" -- NOT that Cufon was EVER worth trying to use in the first place given it's reliance upon scripting, massive bloat and over the top FOUC. It was a bad idea when it was introduced, and has no place on websites now that @font-face is real world deployable.
My advice would be to convert the fonts to ACTUAL web formats using a service like Font Squirrel, and use that instead of Cufon... especially since it not only works in all modern browsers, it works all the way back to IE 5.01 without plugins.
Another way to deal with it is to not use really large fonts -- the larger the font file the longer the delay before it finishes loading.