You may check your version like this:

<?php
$v = Imagick::getVersion();
preg_match('/ImageMagick ([0-9]+\.[0-9]+\.[0-9]+)/', $v['versionString'], $v);
if(version_compare($v[1],'6.2.8')<=0){
   print "Your ImageMagick Version {$v[1]} is '6.2.8' or older, please upgrade!";
}
?>

Since it's not clear from the documention what this function returns:

print_r(imagick::getVersion());

Array
(
    [versionNumber] => 1632
    [versionString] => ImageMagick 6.6.0-4 2010-11-16 Q16 http://www.imagemagick.org
)

The getVersion function for the ImageMagick class returns an array, with two values, one linked to the key value of 'versionNumber' and another linked to the key value of 'versionString'.  The version number provides a simple integer, while the string provides the actual x.y.z version number, the release date, and the URL for the ImageMagick site.

Today (May 8, 2012), when calling this function, the results are...

Number: 1654
String: ImageMagick 6.7.6-8 2012-05-02 Q16 http://www.imagemagick.org

Last month (around April 15, 2012), when calling this function, the results were...

Number: 1654
String: ImageMagick 6.7.6-1 2012-04-09 Q16 http://www.imagemagick.org

So, I'm not sure why the "Number" value has not changed, but the "String" value is definitely responding to changes in time.

The code for producing these results:

<?php

        // Author: [email protected]

    $imagick_version = Imagick::getVersion();
    
    $imagick_version_number = $imagick_version['versionNumber'];
    $imagick_version_string = $imagick_version['versionString'];

    print("Number: $imagick_version_number");

    print("<br>");

    print("String: $imagick_version_string");

?>