Ah, unfortunate. Hope a fix can be found. I remember devs thought they wouldn't be able to fix it for the Evo but eventually it was fixed.
They also use an app called HDMwIn to get it working. Can you try using that app and seeing if it works? Miui is based off of CM7 and CM has the code written in to it to use the app. Found this on another site: "You forgot to mention the better, FREE alternative called HDMwIn, from the same folks who brought WiMAX to non-Sense ROMs (TeamWIN). The biggest difference (besides being free) is that HDMwIn requires a special kernel which the app can install for you. CyanogenMod 7 already has/will have the HDMI code merged into its included kernel."