I can mount several types of OS X disk images from an AFP (TCP/IP) network share, or even from an iDisk (slow obviously), but not sparsebundle encrypted disk images. I can only mount those from a local drive.
This to be a limitation of both 10.5 and 10.6 (Leopard and Snow Leopard). I found a related Apple Discussion thread and an older 10.5 thread on NAS mounting.
I was trying to do this as a workaround for SuperDuper not supporting drive encryption. I had hoped to first backup locally to a sparsebundle encrypted image, then move the image to a shared drive which I hoped to then update over the WLAN. That way I could safely rotate my SD backups offsite.
This doesn't work. We really need SD to support encryption.