ADB is working, fastboot is not (HTC Desire Z)
From your first quoted block, I assume your phone is booted up normally (adb devices lists it as "device"). Fastboot cannot find it there. Try adb reboot fastboot, then check again when your device has booted up into fastboot mode.

See also:


How to Boot Android Devices in various modes (Fastboot/Recovery/Download)
fastboot intro (CyanogenMod Wiki)
FastBoot Mode (KingoApp)
Know Your Android Tools: What Is Fastboot &amp; How Do You Use It?
our fastboot tag-wiki