If you don't have DangerSPL, OpenEclair will fail to install as it doesn't have enough room to place its files.
However, if you do have it...
I have noticed this with some of the 2.1 ROMs. I think the reason is that, since we flash using an "updater", it wont wipe the partition before flashing. This leaves the possibility that there are old files with different permissions that prevent the new ROM from properly taking.
Boot your phone into fastboot (for the G1, hold the camera button while powering up), set it into usb mode (by the "back" button, I think), then use the fastboot tool to clear the boot and system partitions:
/fastboot wipe boot
/fastboot wipe system
This will clear the "ROM" component from the phone, and you can then go and flash whatever ROM you want. Make sure to wipe the data partition via recovery.
If, after wiping the ROM and rebooting, the phone is stuck at the carrier screen and says "Fastboot USB", that means it tried to boot the usual way and found no data in the boot partition. You need to boot into recovery and flash a ROM. Using "fastboot reboot" or the reboot command from the phone's fastboot screen will cause this. Use the phone's fastboot "power off" button ("menu" in the G1), then boot it into recovery.
Also remember that any install that fails after the "verification" phase will destroy the existing ROM, since you will be left with a partially installed ROM on top of the older one.