If you're on the same lan it's as simple as going to the IP address of the local server on your phone.<p>I recently tried exactly this. Running MAMP on my MacBook pro, went to 192.168.0.5 (my MacBook's IP address) on my iPhone while connected to the same wifi as the MacBook and there was the MAMP localhost page.<p>The only problem is (as far as I know) there's no way to edit /etc/hosts on an iPhone (although I imagine you can do it on Android), so it has to be IP based.
If you are using an iOS device and your server is on the same network, you can use Bonjour/ZeroConf to quickly access it. For OS X, you can set the hostname in System Preferences > Sharing. Then just access hostname.local from the iOS device.