Hi All
As Jelle predicted a reboot of Linux gives no control over the CM15a. Need to work with udev rules...
Since I'm a Linux newbe Ron's tip (using chmod +S to the binary file (X10Cmd)) sounded more attractive

This is what I did:
ls -l X10Cmd
-rwxr-xr-x 1 root root 94976 2011-11-06 14:34 X10Cmd
sudo chmod +s X10Cmd
ls -l X10Cmd
-rwsr-sr-x 1 root root 94976 2011-11-06 14:34 X10Cmd
This does not give the expected result. Still no control over CM15a from the command prompt.
After extensive digging on the Internet for a 'how to' on udev rules found a tip to fix this problem. See steps below:
Step 1: Open terminal session (Ubuntu 11.10: Ctrl+Alt+T)
Step 2: Find out hardware details of the CM15a connected to the usb port:
lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 002: ID 413c:8140 Dell Computer Corp. Wireless 360 Bluetooth
Bus 006 Device 002: ID 0bc7:0001 X10 Wireless Technology, Inc. ActiveHome (ACPI-compliant)
Bus 007 Device 002: ID 0b97:7761 O2 Micro, Inc. Oz776 1.1 Hub
Bus 007 Device 003: ID 0b97:7772 O2 Micro, Inc. OZ776 CCID Smartcard Reader
On Bus 006 Device 002 my CM15a is found
Step 3: change to the directory that holds the rules files
cd /etc/udev/rules.d/
Step 4: Create a rules file and copy/past rule below
sudo gedit 10-X10Cmd.rules
# udev rules file for X10 CM15a
BUS=="usb", SYSFS{idVendor}=="0bc7", SYSFS{idProduct}=="0001", GROUP="plugdev"
0bc7 and 0001 are taken from the info at Bus 006 Device 002
Step 5: Save and exit gedit
Step 6: Reboot Linux
Step 7: Test control of CM15a from the command prompt
X10Cmd A1 On
Step 8: test control from within Domotiga
That started working after going into setup|interfaces|X10Cmd to hit save after restoring the existing path to the X10Cmd binary. Sounds strange but true.
Regards Olaf