I figured it out :)
First, I used the following software to ensure that I was generating the proper container string:
http://www.ldapbrowser.com/download.htm
This confirmed that my string was pretty much correct, aside from missing a port, but it just needed some fussing.
The correct usage is:
const string Domain = "SLO1.Foo.Bar.biz:389";
const string Container = @"DC=Foo,DC=Bar,DC=biz";
const string Username = @"sanderso";
PrincipalContext principalContext = new PrincipalContext(ContextType.Domain, Domain, Container);
UserPrincipal userPrincipal = UserPrincipal.FindByIdentity(principalContext, username);