Credit goes to Koush for releasing the official ClockworkMod Recovery images
These are safe to flash via fastboot.
These can be flashed via fastboot using the following commands:
fastboot flash recovery /path/to/.img fastboot reboot
Note: To get the recovery to stick on reboot when Clockworkmod Recovery asks you if you want rename flash-recovery.sh or similar say "Yes", this will allow you to keep Clockworkmod Recovery installed permanently (Unless you flash a different recovery image) when you reboot your phone.