I am not a trademark expert, but I picked up some interesting information about trademarking your apps recently, and thought I would share. The information I read was written by Georga Hay of Freehills lawyers (Australian top tier law firm).
An App is essentially a piece of software for your phone and you don't need an iphone to get one. I use a Nokia and there are plenty of Apps available for it too, although not in quite the same volume.
You can trademark software, and you could also be displaying trademarks via your app. So you will need to figure out what it is you really want to protect with trademark registration. There is a difference between using a trademark as a trademark, and using it for other purposes, like decoration. I won't go into it here, but there are a number of cases around the world that illustrate this point, most recently in Singapore.
When you apply for a trademark, you have to nominate what classification your trademark fits in to. Each different classification attracts a fee. So if you want to register a trademark for printed material, it won't necessary be protected for spirits and alcohol, as an example.
The current international standard for classifications is the Nice Classification published by the World Intellectual Property Organization- WIPO.
You should consult a trademark specialist to ensure you get the right classification for your app.
An App a day keeps the trademarks on display.