I agree that comparing the actual number would be better than comparing a string, but intval returns an integer. Your test above is the same as saying

if (intval($wp_version) >= 2)

In this case, it’s equal to 2, so it runs the 2.8 hook. For more accuracy, use floatval instead:

if (floatval($wp_version) >= 2.8)

BTW, I applied your new lines and received an error. Seems ‘left()’ is not valid. In some other coding I generally check the WP version by converting it to an intval then comparing it to what I want. Something like this should work. Or I should add it works for me.
if (intval($wp_version) >= 2.8) {
add_filter(‘transient_update_plugins’, ‘update_active_plugins’); // Hook for 2.8.x
} else {
add_filter( ‘option_update_plugins’, ‘update_active_plugins’); // Hook for 2.7.x
}

Thanks for the tip though. I just got lazy after digging through the WP core code looking how to implement this code. It was on a Sunday afternoon and I was fighting a nap. cheers.

if (left($wp_version,3) ==’2.8′) {
add_filter(‘transient_update_plugins’, ‘update_active_plugins’); // Hook for 2.8.x
} else {
add_filter( ‘option_update_plugins’, ‘update_active_plugins’); // Hook for 2.7.x
}