This iMac email notification page is linked from their main iMac page. I guess it's better to link to this than to the Apple Store, which only has this to say:
Apple has stopped taking orders for the current iMac as we begin the transition from the current iMac line to an all-new iMac line which will be announced and available in September. We had planned to have our next generation iMac ready by the time the inventory of current iMacs runs out, but our planning was obviously less than perfect. We apologize for any inconvenience to our customers.It's likely we'll hear more about the iMac G5 on August 31.