first off, nice package. :) Having said that, I have a few questions or additions which make this vote contional.
1) It would be nice if you used visibility in all of your code (and not just in phpdoc). Sometimes you did make use of it, sometimes not. No idea why. ;-)
2) Is there a reason why you always trigger_error() in your parser instead of using an exception? If not, I'd like to see exceptions. If this is absolutely necessary, let me know and consider it to be unconditonal.
3) I'd like to see all get'ers prefixed with get, e.g. Net_Nmap_Host::os() -> Net_Nmap_Host::getOs(), etc..
4) Not conditional, more a suggestion - replace is_null($foo) with $foo === null (those calls are less expensive).