Have exactly the same problem with a Mini9: it was running fine with 10.5.8 (and DellEfi), and suddenly got a kernel panic ("Please press the power button..etc.etc"), and then got the same error.
I booted the mini with a retail leopard disk and tried running Disk Utility. It was unable to repair the disk (listed as damaged with volume, catalog issues).
I then got the TechTools Pro 5 DVD image (bought from MacUpdate, but available at "micromat.com") and restored it to a USB disk; then used NetbookMaker on the restored image and was able to boot the mini with the TechTools USB; however, the window that comes up is too large for the mini's screen, so you can't reach the buttons!!
Therefore, I then downloaded the "mirror" cmd line utility from "http://www.fabiancanas.com/Projects/MirrorDisplays/", and installed it in "/usr/bin" in the TechTool USB, chmoding it to 755.
I proceeded to edit "/etc/rc.launchapp" in the TechTool USB and modified the line:
/Applications/.TechTool\ Language\ Selector.app/Contents/MacOS/TechTool\ Language\ Selector
/Applications/.TechTool\ Language\ Selector.app/Contents/MacOS/TechTool\ Language\ Selector && /usr/bin/mirror -f
This basically allows me to boot the mini with the TechTool USB and an attached monitor (use Fn-8 to activate both monitors while the Apple logo is showing during boot). The second monitor does allow access to the TechTool buttons at the bottom of the screen.
However, when pressed, they do nothing (not sure why, still working on it).
Therefore, I will be taking the SSD
(Dell 32GB) out and plugging it through USB into a iMac to run TechTool from there and see if the catalog/volume corruption can be fixed, rebuilt, or data salvaged.