it does not appear that Discovery HD Theater is going anywhere.
in this image, posted by scott, you can see that BOTH discoveryHD and discoveryHDtheater are shown as HD channels.
Evidently, they are going to get DiscoveryHD up first, and probably discontinue HDTheater later when it is duplicative.
Discovery has several other HD versions of SD channels planned for this fall, see Discovery HD threads.
PS The date is August 15th, due to the existing Voom MPEG2 channels changing to MPEG4 on August 15th, thus providing enough extra bandwidth for the new channels.