I think you need a custom header in your soap posts.
you can use this to specify the credentials of the user from whom the action is performed.
This is a pretty advanced subject of this ...
2D00--solution.aspx" > http://weblogs.asp.net/avnerk/archive/2006/04/26/Adding-custom-headers-to-every-WCF-call-2D00-a- solution.aspx
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