I personally would really like to see this. It is getting quite old seeing the forums filled with code that isn't coloured(Canadian way of spelling it) unless the poster does it themselves.
There is an opensource php script colouring called
Geshi. All we would need to do is have like [code=javascript] or [code=xml] and it would code based on that. Obviously the colours would need to be changed but Geshi is really powerful seeing how I know how lazy DZ is I found this.