The browser-based form executes in the context of the server so the only way to get the IP is to write some code that executes on render to ask IE for the IP address of the local computer. Not sure that is going to be very easy, if it is even possible. Certainly, web sites do this, so there's got to be some code out there. The issue is having the InfoPath Form Services execute it. Probably doable but requires some investigation.