What is the current state of the law on the Miranda warning?

"The Fifth Amendment prohibits use by the prosecution in its case in chief only of compelled testimony. Failure to administer Miranda warnings creates a presumption of compulsion. Consequently, unwarned statements that are otherwise voluntary within the meaning of the Fifth Amendment must nevertheless be excluded from evidence under Miranda." (Oregon v. Elstad (1985) 470 U.S. 298, 306-307.) However, "[i]t is an unwarranted extension of Miranda to hold that a simple failure to administer the warnings, unaccompanied by any actual coercion or other circumstances calculated to undermine the suspect's ability to exercise his free will, so taints the investigatory process that a subsequent voluntary and informed waiver is ineffective for some indeterminate period. Though Miranda requires that the unwarned admission must be suppressed, the admissibility of any subsequent statement should turn in these circumstances solely on whether it is knowingly and voluntarily made." (Id. at p. 309.)