Pretty sure the HDMI is output from playbook only.
The only hope I see is if an app is developed for your particular camera using the USB as input to playbook, or a wifi hotspot of some kind attached to the camera.
I hope someone develops a solution for this. Of course a micro SD card could be taken from the camera and put in your phone and bridged but too much agony.
IPad has some kind of camera connectivity dongle, not sure how that works or if something similar could be developed for Playbook.
Keep the thread alive!
If it's HDMI compliant it is in-out by nature: the source (PB) has to know on what kind of device it will display its signal (SD, 720p, 1080i, 180p ...). Now the point is: we have to know if PB hardware is able to decrypt the signal (HDCP protection matters, mainly).
But you're also right, USB can do the trick for pictures ... but usb host is not there ... yet.
EDIT: ooops, and my favorite horse : DLNA ... mi amore
