I'm seeing big brands moving into the top spots on some niches where they hadn't ranked before... but the results don't yet seem stable. I'd always (ie, for the last 8 or 9 years) felt that some niches were vulnerable to major brand competition if the major brands intelligently targeted them. In some cases, that now appears to be happening.
The targeting, though, is more than just SEO... the brands are actually paying attention to those niche areas. Sort of like specialized food shops being replaced by expanded deli counters in some savvy supermarkets, or chains created to cover those niches.