According to multiple sources close to Microsoft,WIndows 7 RTM (Release to manufacturing) is set to be available on July 13th.Microsoft has not made any official statements.Basically the RTM version of any software is very significant because it is the version most similar to the official version.
RTM is simply for manufacturers to make sure all their hardware/peripherals are compatible with the new OS.  The official launch of Windows 7 is still October 22.  Another reason the RTM date is important is because it will definetly be  the version that contains all the major fixes to bugs.