Just installed OS/2 ECS 2.0 RC5 as guest on Ubuntu hardy.
Works perfectly.
Follow mouse.sys workaround explained in this forum to use mouse pointer integration.
Once you have mounted parrallel tool drive you have to:
1) install the mouse package included in \drivers\mouse\os2 by running install.cmd and not the one included in \drivers\mouse\ecs12
2) move mouse.sys only (and not the other dll files) from \drivers\mouse\ecs12 to c:\os2\boot
3) reboot the VM
this worked for me.
The only problem left is that the caps lock is not working, or better, it's working but to led appears on the keyboard.
After this just copy in os2\boot the file mouse.sys included in \drivers\mouse\ecs12