According to an email (above) that Apple is sending out to their iWork.com users today, they are going to replace the iWork.com public beta with iCloud on July 31st of this year, though you will not be able to view any of the documents from the existing service. This is further evidence that the documents integration of iCloud in Mountain Lion will be much better than they are currently in OS X Lion. In addition, it shows that the Mountain Lion release date may be close to this one.
Apple advises that you switch all of your documents that currently reside in the cloud over to their iCloud system to continue using them, but you have quite a while before anything will actually happen to them. After July 31st, iWork.com will most likely redirect to iCloud.