Is there any software that i can use on my ubuntu laptop to write SMS on my Windows Mobile 6.1 HTC Touch Diamond?
It should be possible to combine SynCE tools and the WinCE sms program for sending sms messages:
You'll see that there is a command for running programs on your wince phone. And if you can figure out what the sms tool is called, you should be able to call the program and send a message using it. If you can't, you may have to write one yourself:
Overall, Windows Mobile phones aren't supported natively in Ubuntu and you need to get in touch with the developers of SynCE to get functionality if you want it to be easier to use or integrated with other systems or phones.