G1/Dream running CM6 - black screen, won't boot, can't reset
Since you have CM6 flashed already, try booting the phone into the bootloader (fastboot). If you can get in there, you can rescue your phone with fastboot erase cache; If that doesn't work, reflash CM6. This will cost you your data that's not on the SD card.

Opinion: Having had a bunch of G1s (all secondhand), they are not the most reliable phones I've owned. I have had one turn into a brick (permanently!) after leaving it in a hot car (next to my Magic, same chipset, which survived) for 30 minutes.