We take out travel insurance before we go, and there are very good hospitals in Chiang Mai and Mae Hong Son, as well as fracture clinics in other smaller towns and villages. Also there is a good mobile phone network in most, but not all areas. We use 2-way radios between guides & riders, which can work up to 5 kilometre distances.
HOWEVER - we will be in pretty remote locations - you cannot simply phone the AA if a bike breaks down, or an ambulance if someone breaks a leg.
If we have bike trouble, we try to fix it, or tow it out (one XR spilled it's guts on the last day in January '03, so we had to tow it 160 kms back to Chiang Mai).
If anyone gets hurt, we call up our 4x4 and take the person to hospital. BUT - this could be half a day's drive away - so prudence and care is the order of the day. We haven't had to do this yet, and to be honest I don't want to have to test the system.
UPDATE. March 2006 - the "system" got well & truly tested. One broken foot (on the first day!) causing serious delays, and one dislocated & slightly broken shoulder a few days later. The latter incident required rescue by our back-up vehicle from a rutted trail 20km deep into the jungle. As anticipated, we rescued the fallen rider, and took him to hospital, but judging by the expletives in Russian, I don't think he felt it was a very enjoyable journey.