New iMac in September

This time it's official because for the first time in computer history a company admit publicly to be late in their promises:

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 planned to have our next generation iMac ready by the time the inventory of current iMacs runs out in the next few weeks, but our planning was obviously less than perfect. We apologize for any inconvenience to our customers.

Source: Apple