you are relying on the telnet service to be running on the remote host. This typically would work fine for all flavours of *nix

Not while I'm in charge of the *nix machines! (But normaly nmap wont detect the os-type on these either, thanks grsecurity, Update: though this is likely to change, as far as I know nmap, it will be able soon).

regards,
tomte

Hlade's Law:

If you have a difficult task, give it to a lazy person --
they will find an easier way to do it.