Ars Technica says that they've heard from an inside source that Microsoft will start making Core and Premium systems with HDMI ports after the exisiting stock has sold through.
'According to my source, Microsoft wants to cut through all their existing inventory of hardware. They will then quietly "soft launch" a new version of the Core and Premium hardware in late August or September that will include an HDMI port. It's also expected that this new hardware will feature the 65nm process for the CPU and quieter DVD drives. The new hardware will ship in boxes very similar to what we saw on store shelves now, but there will be a small "call out" on the box so you know you're getting a system with the HDMI connection.'