If your using the PS3, enable BD 1080p 24 Hz Output (HDMI) under the video settings. If you leave it set to auto it will never output 24fps on the Mits, it must be set to On. If you are using a different Blu-ray player consult your owners manual.
I found the same as ditch did when I tried it a while ago. It displays 50Hz when given a 24fps signal. The HC1500 can't do a true 24fps so it gets confused and switches to 50Hz mode. Projectorreviews.com says the HC1600 does do 24fps though.
I stand corrected. I switched my HC1500 to 1080p/24 and since the PS3's last firmware update it now has a message that says if 24fps is not supported the screen will go blank for x number of seconds. After switching the PS3 says it's support and the HC1500 displays 50Hz.
I did notice that 1080/24 isn't as sharp as sending 720p to the HC1500.