Thanks to ContractsProf Blog for the following: 

In late June, the Missouri Court of Appeals addressed the legal enforceability of a program adopted by Hallmark requiring employees to arbitrate employment disputes. The court held that Hallmark's ADR program did not constitute a contract and that there was no consideration to bind the employees to the promise to arbitrate claims.

The employer's arguments in favor of enforcement in this case were very much like those argued by O'Melveny & Myers here in California with the same result in the Ninth Circuit  --  the employee was not bound by an agreement by continuing to work after all employees were notified that their continued work for the company would constitute consent to being bound by the arbitration provision.

Check out the ContractsProf Blog analysis here.  We particularly like this comment by the Court:

The idea that an employer can create any legal contract it dares to create (based on a condition of at-will employment) cannot be sustained upon reflection. Imagine, for instance, an employer publishing a memo to employees stating that:

Anyone who continues to work for us through next Monday will be conclusively deemed to have agreed, as a condition of remaining in our employ through that date, that you will contribute twenty dollars per month over the next ten years to the National Association of Manufacturers (NAM), whether or not you remain employed here during that time. If you do not agree, you will need to resign your employment immediately, because by continuing to work, you are agreeing.

Yes, I did see the Beatles play the Hollywood Bowl in 1964, with an emphasis on SEE -- couldn't hear a thing!  Just a little nostalgia for my boomer readers.