First, to state the obvious, the new rules only apply to cases in federal court, although no doubt the new Federal Rules of Civil Procedure and their commentaries will also be frequently argued in state court e-discovery disputes.
Second, the new FRCP certainly apply to all cases filed in district courts on or after December 1, 2006, the date the new rules went into effect. As to federal cases filed before December 1, 2006, the Supreme Court said they should apply “insofar as just and practicable”. See: http://www.supremecourtus.gov/orders/courtorders/frcv06p.pdf
Thus, under the Supreme Court guidelines, if a case was filed before December 1, 2006, it will be up to the district court to determine on a case by case basis whether application of the new rules will be too burdensome, or result in undue delay. If a court determines it is impractical or unjust to use the new rules, then the old rules will apply. This will almost certainly be the case where discovery has already closed or is in the final stages.