I have a postdoc offer for one year in Canada. The postdoc is in computer science, and it includes research and teaching.
Is this considered as an arranged employment for the express entry?
It can be. Subject to conditions. If this offer is only for one year, non extendable, then no.
see this and decide yourself.
http://www.cic.gc.ca/english/immigrate/skilled/offer.asp
From reading the link it says that the job offer must be "for at least one year after we issue your permanent resident visa".
Does it mean that a postdoc offer while still not a resident is not considered as an arranged employment?