Sure. Everything is possible:
http://gkaindl.com/software/arduino-ethernet
The Arduinio will run a DHCP client and will receive it's IP from a DHCP server. That'll be either the router of the existing network you will connect the pocket router to (pocket router in Bridge/Client mode) , or it will be the pocket router itself (in AP mode).
Then of course the IP address that will be assiged to your Arduino is subject to change. So you need a method to tell your the application on your Android device. Apples implementationon of ZeroConf technology called Bonjour can do that. Bonjour on your arduino will register a service on the network that you application will be able to pick up. Your network devices (router, Android) need to support Bonjour though.
Most Routers do, but I remember that some time back getting Bonjour is not easy on Android devices. But it's been quite a while since I looked at it alst and it may have changed. Naturally that all works flawless if you stick with Apple devices.
If you intend to use the pocket router in AP mode anyway the DHCP/Bonjout approach is really overkill. Hard coding an IP address that you only use in the Arduino <-->pocket router <--> Android network is really not an issue.