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the FAQ is useful to read.. also some themes have this setup for user login in the header, you could download 1 of those and then view the code and add that part to your theme..
*EDIT* i think also 1 of the themes that comes with XOOPS has that login section in the header.. either phpkaox or x2t theme.. if u look at their theme.html you should be able to see the way it works :)