Here's a sprinkling of MTB races in Dec2008 to Feb2009 ... translated from the ThaiMTB Race Events section. Links lead to the ThaiMTB forum page.
Dec 14, 2008 -- Phraputhabat, Saraburi -- race / caravan to visit temples -- Contact: Kh Witoon วิฑูรย์ 0819944082. Link.
Dec 17, 2008 -- Patong Beach, Phuket -- Phuket Carnival Adventure Race Challenge Series 2008 -- more details here. Update Dec 22 -- here's the link to the race results. link.
Dec 19, 2008 -- Bangphra, Chonburi -- community mtb race - start/finish at the Bangphra Local Community Administration. Details: contact Kh Chaovalit เชาวลิต 081-4282135. link
Dec 20, 2008 -- Burachart Army Engineer Camp, Ratchaburi -- mountain bike race with mini-marathon - start/finish within the military camp. Details: contact 089-9162005 . link
Jan 18, 2009 -- Pathumthani. somewhat sketchy information, but a race calling itself as Adventure The Series -- some type of an adventure race, but did not elaborate further. no start/stop info. Get the details here - 0840719592-027189581-2 Khun Nid. link.
Jan 17, 2009 -- Jombeung, Ratchaburi -- annual marathon / mtb race -- here's the link.
Jan 25, 2009 -- Kamala, Phuket -- organized by Khao Kamala MTB club -- contact: Gopiak 084-6890691 อภัย บุหลัน (โกเปี๊ยก) -- details here.
Jan 25 as well but in Udonthani. -- sponsored by the Tesabal. Contacts - Kh Tone 081-261-3210 and Kh Loh 081-975-0015. Link.
Feb 8, 2009 -- Ban Bueng, Chonburi -- organized by Khlong Kiu MTB. Details Kh Chai 089-9394840. Map and class categories detailed in the forum. here's the link. Pre-ride anyone? This one's not so far from BKK.
Here's another one on the same day Feb 8 in Sakhon Nakhon. This seems to be the 3rd installment of this yearly event. Contacts: Mr Teung 08-3670-2458 and Kh Toi C4 08-7636-7578. Link.
Feb 15, 2009 - Jomthong, Chiangmai -- organized by Jomthong Bicycle fitness Club. Details - phone no 053-341134. Link is still just a teaser. Site claims more details to follow later. Link.
Feb 22, 2009 -- Pattaya Triathlon -- organized by the city and Pattaya Triathlon. Finally, a link with English texts. you can check out the details yourself in -- link. Contact: Kh Prayoon - 081-9962323.
Sorry that i had to do the shortcut on the translation. Too pressed for time to do that in this blog. Hope, though, that this gets you pointed in the right direction.