This may be a stupid question, but does your display accept 1080P? Have you succesfully used 1080p out of the AVR before?
If 1080i works, then I assume that your HDMI board is OK.
Can you hook up a blu-ray directly to your tv to see if it still works with 1080P _
i.e. is this a tv/projector problem or your avr
Also, apparently HDMI cables can be very finicky and if you have an older cable, it may not be able to handle the higher bandwith of 1080p. Also, HDMI cables can get partially broken with a bent pin and then stop working at the higher reolutions; or the HDMI cable may not be fully seated in the hdmi slot.
Try another HDMI cable that you are sure is functional.
Also, does your tv/projector accept 1080p 24hz or just 60hz - maybe a setting was changed is this regard.
Out of curiosity, do all other resolutions work?
Also, check out: http://www.avsforum.com/avs-vb/showthread.php?t=1016297
It is the thread for firmware updates in the onkyo. There has only been one or two and I just can't remember now if it invloved the HDMI output.