Actually, not. Any rule in a splat book that conflicts with a rule in a Core rule book is decided in favor of the Core rule book.
Not really. Dnd adopts a "the later rule supersedes the rest" approach. If a rule in a later splatbook contradicts that of the core rulebooks (or that of an earlier splatbook), the latter splatbook takes precedence. This is the whole concept behind books such as MIC and Rules compendium, which serve to clean up perceived flaws/shortcoming in the core rules.
Thus, a warblade could wear mithral fullplate with no penalties despite not being proficient in heavy armour since mithral fullplate is treated as medium armour for all intents and purposes.
