Hi,
It is possible, but you have to deploy a web service on the sharepoint portal server, and you'll be using that web service to get the username of the user logged in. I dont know any other way of knowing w/o deploying the web service.
Look at the following article, I believe it'd help you:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/odc_SP2003_ta/html/sharepoint_integrating_sps_and_infopath.asp
-Attari-