I merge all my contacts (skype, Gtalk, and googleVoice) the merging is only on the n900. If you delete a contact on the n900 (Skype, or google) It will delete it from the server. If you have two way sync going. (Skype/Gtalk is two way) I know....I have done it before...
http://talk.maemo.org/showthread.php?t=41857