Forgive the ignorance, but what does HDMI enable one to do that can't be done already?
HDMI does a lot of neat things.
DisplayPort, as a standard, has been around for several years now. Outside of Macs, its basically failed to be adopted. Even PC manufacturers that include DisplayPort throw in an HDMI port for good measure.
Apple doesn't allow audio over DisplayPort. HDMI does video and audio over one cable, and has more bandwidth available for audio than DisplayPort.
HDMI 1.3 supports 2560x1600 over a single link cable, with 8 channels of uncompressed LPCM audio.
HDMI 1.3 also has a "mini" connector thats about the same size as the current mini DisplayPort connector.
More importantly, HDMI is everywhere. Every respectable computer display made within the last half decade has had HDMI connectivity. HDMI has been standard fare on PC notebooks with dedicated graphics for the last 3 years, with Intel GPUs coming on board in the last 2. Even some netbooks include HDMI. Desktop GPUs have had HDMI connectivity for many years now. Apple is basically the only computer manufacturer that doesn't support HDMI. Even the Zune HD supports HDMI!
Let's just hope Apple does this right. This would be fantastic for the Mac mini. But Apple has a thing for giving one new feature while taking away another. It would be very disappointing, but not entirely surprising, if Apple went back to Intel integrated graphics for the Mac mini. Last year the Mac mini finally got its first respectable GPU since the G4 days. Let's hope Apple doesn't take that away.
Let me say this just to anyone from Apple reading this: a Mac mini with a faster Core 2 Duo, GeForce 9400M, and HDMI would be better than ANY Mac mini with a Core i3, Intel GPU, and HDMI. The GMA 950 Mac minis were a huge step backward from the Radeon 9200 powered G4 Mac minis. Don't take that step backward again. The 9400M is still faster than any Intel GPU, and a Core 2 Duo and GeForce 9400M would be a better combination than a Core i3 and Intel GPU. A Core i3 might be slightly faster than a Core 2 Duo, but that doesn't matter when you're cutting one of its legs off by pairing it with Intel integrated graphics.