Custom function to change text strings.From:http://www.viper007bond.com/2011/07/13/changing-core-wordpress-strings/

filter_gettext.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function mycustom_filter_gettext( $translated, $original, $domain ) {
 
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        'Register' => 'Sign up',
        'Confirm and go to payment page' => 'Complete registration',
        'No events available...' => 'No upcoming classes at this time...',
        'click here to add a new state/province' => 'click here to enter address outside US/Canada',
        // Add some more strings here
    );
 
    // See if the current string is in the $strings array
    // If so, replace its translation
    if ( isset( $strings[$original] ) ) {
        // This accomplishes the same thing as __()
        // but without running it through the filter again
        $translations = get_translations_for_domain( $domain );
        $translated = $translations->translate( $strings[$original] );
    }
 
    return $translated;
}
 
add_filter( 'gettext', 'mycustom_filter_gettext', 10, 3 );

Just a little heads up to peple running non-english installations: The strings in the $strings array needs to be the original english string, not the one that's already being translated. So, in my case, I wanted to change the phrase "Tilgjengelige billetter", but still had to put "Available tickets" in the $strings array to make it work.