Don't suppose you noticed that this thread is from June, 2008...
Just make sure the ground stays moist, but not wet. Give it a few weeks. Warm sunshine will help so if the weather isn't the best it may take a little longer. Of the 8 rhizomes I planted 3 years ago, only one didn't come up. I think there was a split in the rhizome; that, combined with a wet spring, allowed rot to set in. The last rhizome to come up did so at 4-5 weeks. I also think I planted mine a bit deeper than necessary.