G1 will get Donut and Eclair. Google knows the install base of that device is large enough and the phone is still less than 1 year old.
1.6 - Donut should be out soon. The SDK release is an indication that the OTA Update is coming shortly. After Google, Tmobile runs testing on their end, then starts with a tiny fraction of the userbase and slowly rolls out the update. The keyword is slowly, in case there are issues.