How can I connect Arduino to a smartphone (Android)?

One of the reasons I decided to develop this project was the needed to research about implementing two actual and strong technologies that are taking more and more importance nowadays.

Android is one of the most important open source OS related to mobile devices, is a platform that is growing by leaps and bounds, allowing to create many applications systems in order to interact with real world.
Arduino is, also an Open microcontroller platform which allows to interact with a bunch of sensors, so these devices can be easily implemented.

Now I'm going to present different options that I found on several blogs to implement the connection:

1.- Audio Port:

• Sensor Data to iPhone Through the Headphone Jack: http://bit.ly/mcmQN4
• Project Hijack: http://bit.ly/enJiUX
• iPhone Thermometer - Temperature Logger: http://bit.ly/c7RNNe Important: FSK Link
• SoftModem Interface Board: http://bit.ly/nMahcu
• SoftModem tested on Android: http://bit.ly/pJFurJ

• Additional OpenHardware: SOFTMODEM BOARD http://bit.ly/or5irV

2.- USB Port:

• Connecting Nokia phone to Arduino (USB cable): http://bit.ly/pqp3Pn
• Experimenting with Android and Arduino: http://bit.ly/hr2Ebt

3.- Bluetooth/WiFi

• Amarino: http://www.amarino-toolkit.net/index.php
• Zigduino: http://www.logos-electro.com/zigduino/

So, I decided to perform the connection through the audio port, because it's an universal port that exist on all the different devices that support Android, so this would allow phones to form the basis for many health and communications applications turning the mobile phone into a scientific data collector data or an environmental tool by connecting low-power sensors.