i would use vito sms, but i sadly don't know how to set up as default, if anyone would explain how to make vito default sms, I would love that haha =]
vito can be bough here
http://iwindowsmobile.com/threaded-sms-messaging.html
PM me if you want to know more about getting vito =]
edit: ooo poster above beat me to it, but still PM me if you would like to =]