There are plenty of solutions for making your WordPress mobile friendly. Most of them redirect to a separate mobile friendly theme. However this may be overkill for certain sites which just require a few minor changes to look right on a mobile device. This solution could be used for example to load a separate style sheet for mobile devices or to only show a javascript photo slider on desktop PCs.

First download Mobile_Detect.php from http://code.google.com/p/php-mobile-detect/

Then open the template file you wish to include your mobile specific changes eg header.php file in a text editor and add the following:

<?php
include("Mobile_Detect.php");
$detect = new Mobile_Detect();

if ($detect->isMobile()) {
 // code to run for mobile devices
} else {
 // code to run for desktop browsers
}
?>

If you just want to detect certain mobile devices like android phones then you can use the following:

if ($detect->isAndroid()) {
    // code to run for the Google Android platform
}

Available methods in Mobile_Detect.php are isAndroid(), isAndroidtablet(), isIphone(), isIpad(), isBlackberry(), isBlackberrytablet(), isPalm(), isWindowsphone(), isWindows(), isGeneric()