With all the reports of Yahoo mail having an outage on Sprint it seems to actually be a more widespread problem with other carriers experiencing problems. My Yahoo account on Cingular/AT&T was also problematic due to this outage.
RIM has posted a KnowledgeBase article that claims if you remove and reintegrate the Yahoo account to your BlackBerry things should return to normal. They claim that a recent security change implemented by Yahoo! Mail has caused this service interruption for some BlackBerry Internet Service subscribers. Check out the KnowledgeBase article for more info if you need.