I just ordered the Parallax RFID reader, should arrive in a few days. I'm looking to see what my options are for writing RFID tags as well. I'd like to duplicate a couple tags I have, rather than have to change the controller code to authenticate more readers.
You can only write to certain RFID tags, not all tags have that ability.
One of the problems with duplicating a tag, is that the id of the tag is burnt into each tag and can't (normally) be changed. The writeable tags let you change the data in the tags memory, but not the id. If the reader uses the tag memory for authentication then you can duplicate tags, however if it uses the tag's id for authentication, you can't duplicate tags.
The writeable tags let you change the data in the tags memory, but not the id.
No that is not true of the 125KHz tags, there are writeable tags that allow you to clone an existing tag.