As from the 16th of December 2009, Nok Air will be relaunching their Udon / Chiang Mai route.

There will be a daily flight from Udon Thani to Chiang Mai departing at 11am and arriving in CM at 12.20.

The return leg from Chiang Mai to Udon will depart CM at 9.20am and arrive in UT at 10.40am.

For booking and further flight info, here is a link to their site:
http://www.nokair.com