This week's book giveaway is in the Other Open Source APIs forum. We're giving away four copies of Storm Applied and have Sean Allen, Peter Pathirana & Matthew Jankowski on-line! See this thread for details.
Probably what you need is a middle tier between your web service and your j2me app. The middle tier can be a java servlet. The j2me sends the request to servlet through http post/get, the servlet invokes the .net web service, retrieves the result and sends the http response back to the j2me client.