I've always heard of it as Ishbal, but reading Serpent's Cross' scanlations of FMA, in chapter 48 they say the Perfect Guide for FMA says the Romanized word is Ishval. Is it another case of the Japanese saying, "Hell...what's the difference between 'v' and 'b'?" or is there anything else? Thanks!
the reason for this difference is viz media's translation and also how the word is written in japanese lit. ishuvaaru. this should be translated into english as ishval, however, due to their being no noise for v in japanese it is pronounced as ishubaaru. this is why it is sometimes fansubbed as ishbal and sometimes translated as ishbal.
but really, it can just come down to opinion because its pronounced the same way in japanese ( the original format) no matter which spelling you take. although my preference is to write it out as ishval, but both are good as i get the meaning.