According to Dean Takahashi of the Mercury News, Microsoft is developing a new motherboard codenamed "Jasper." The upgrade promises to shrink the ATI graphics chip to 65nm and reduce the size of the memory chips as well. If true, this could reduce build costs and the heat problems that have plagued the 360. Although Takahashi notes that Microsoft believes they already have the heat situation under control. The upgrade is expected in August of '08. [Kotaku]
What are you talking about? They have the Core, Premium and Elite. the only difference between them is the cables and storage, that is hardly an issue. Adding HDMI and core revisions are not new skus. Special editions and colors are not skus, they are imited release editions. All systems go through hardware revisions like this, but before now no one ever talked about it. Both the PS2 and the XBOX had many system revisions.
As for why it takes so long you have to realize that development and production time aside, and all the red tape, any hardware revision requires extensive testing of release titles to ensure the revision does not break compatibility.