Hey Jonathan,
I manage Howcast's Emerging Filmmakers Program -- I just want to let filmmakers who are curious know that we actually don't use stock footage anymore, unless we buy it outright. It's just to iffy even if it is in the public domain. One reason (among others) you often don't have the rights to redistribute content that's in the public domain. Hope that makes sense. If anyone has questions, please email me at emergingfilmmakers@howcast.com. Thanks!
HeatherMenicucci