Actually this is more a Web Services tip, but what the heck -  When using the WSS Web Services, you will need to bind your proxy to the asmx file in the _vti_bin directory of the site you want to update. Meaning that to work with multiple WSS sites, you need to be able to dynamically change the binding at runtime.  Luckily that is very easy to do - just change the .URL property on the client-side proxy and point it to the correct site. The URL should look like the following :

http://Server_Name/sites/[Site_Name]/_vti_bin/[ServiceName].asmx