Some users think they are. Microsoft says they don’t think so but they’re checking, just in case. We’ll see soon enough.
Security firm ThreatTrack Security Labs today spotted that certain Bing ads are linking to sites that infect users with malware. Those who click are redirected to a dynamic DNS service subdomain which in turns serves the Sirefef malware from 109(dot)236(dot)81(dot)176. Update: Microsoft is investigating, see statement below.
ThreatTrack notes that the scammers could of course be targeting other keywords aside from YouTube. The more popular the keywords, the bigger the potential for infection.