I was reading Darren's blog I think and he said there that it is best to have a domain name for each blog you start. That way, if a blog goes down in the SERPs the others won't be affected. My questions is: would putting them on separate subdomains counter this problem? Blogger seems to show that they're ranked independently.
Yes. They are viewed as separate web sites which makes them harder to rank than a sub folder, I believe.
Perhaps you can use a subdomain+subfolder Yes, domains and subdomains are treated entirely separately. Just as much as www.yourdomain.com and yourdomain.com (no www) are considered entirely separate.
Ditto what sadcox66 wrote - in most cases, you actually want to 301 the www and non-www so only one is seen.
Subdomains are viewed as search engines as independent domain names, and treated like that also, thus ranked independently. I believe that directories on a server are also treated like seperate websites, also.