There isn't an xmpp equivalent for .Net (that I know of) and you really don't need one either.
For mobile apps you wont be running an iis server on the devices, so you make the app able to run directly on the device pretty much like you would a program running on a system.
For the intercommunication you'll probably want to use WebServices for the messaging between and that would require a Windows based web host on your domain, of which would have iis and can run .net websites/webservices from; your mobile app would just consume the chat WebService(s) you create.