This article by Phil Loree provides a great overview of federal cases addressing claims that a party has waived its contractual right to pursue arbitration, with special attention paid to whether a showing of prejudice is required. Mr. Loree suggests that a split in the circuits makes this issue appropriate for determination by the U.S. Supreme Court. At minimum, those facing a (potential) claim of waiver will find the article to be a great starting point for research of applicable federal cases.