In the context of this particular forum, there's little difference to any other type of testing. You're not interested in the client side operations, only communication with the server - if your app is making calls over HTTP then off you go.
This is where a cultivated architectural eye will be your friend. Are you trying to answer questions on application performance or on application performance over a particular bandwidth link associated with mobile? These are really two different questions and require two different approaches to test.
If you are talking about a Mobile app that is web based (eg.. you use the mobile browser, like Safari on an IPhone), then you should be able to test the app using the Web/HTTP protocol in LR.
If you are talking about a Native Mobile App... good luck. The only way I have seen to do this so far is to use one of the 3rd party add-ins to QuickTest Pro. This means you will create a QTP script, then run it in LR using the GUI virtual user. It will be hard to do a high number of VUsers because each GUI Vuser will require a separate generator.