I have been using Yahoo BOSS for little time. It is an easy API for search but spelling suggestion support is really not that robust. Do folks around here have any idea of getting better spelling suggestion on BOSS.
Since you originally asked your question it looks like they added a "Spelling Service".
http://developer.yahoo.com/boss/search/boss_api_guide/Spelling_Suggest.html
Give it a look to see if it will accomplish what you are trying to achieve.