Fastboot setserial command I noticed that a lot of descriptions of fastboot mention a command named setserial which not available in the current version that comes with Android SDK. 

You can see the complete list of commands including setserial, for example here. The missing setserial command is not the only command missing in the current version.
The setserial command sounds very interesting for some tests I want to perform. 

Does anybody has experience with that command and know where to get a fastboot binary that supports it?

Furthermore it would be interesting if current Android devices still allow to change the serial - may be this was only possible on the G1 and Nexus One?
The option has been moved to the oem subset of fastboot functions. Since every vendor is using their own proprietary ways to handle serial numbers. So there is no universal solution available. It is not a matter of finding a "fastboot binary that supports it". Fastboot binary  just sends commands to the device's bootloader. And "setserial" commands are proprietary.