I have the exact same issue. I followed this guide step by step:
Once I restart, the "ignore ownership on this volume" check box becomes unchecked. Not sure if this is an error with the new operating system (Lion) or if there is some other process forcing the setting.
When you mount a diskimage the filesystem is mounted noowners (ignore permissions) but the operating system honors the permissions on the diskimage file (This is a feature). If you would like to verify this then mount the diskimage then open the Terminal.app and type the following->
Note within the parentheses, you will see noowners with mounted by so and so. The first step is to change the posix permissions on the diskimage file or add an access control entry (I see that the ace does not seem to work). Posix permissions can be change in the Terminal.app or by a third party gui app (not the Finder).
If the diskimage is mounted by the root user then the operating system honors noowners (ignore permissions). This is complicated to implement. Off the top of my head, I might try a loginhook to mount the diskimage.