Well, you can try adding line that code, plus one that removes all existing strips, to the Global .ajaxSuccess()
event so that it runs every time an ajax call is complete, but that isn't a terribly efficient method if you're working with many different ajax calls with only a few that actually changes the table.
You can also define a function that you can include with the success
callback of your ajax calls, such as
function stripTable(){
$('table.alt tr').removeClass('bg').filter(':even').addClass('bg');
}
This will prevent strips from being messed up if you insert in new rows into existing tables.
Using CSS is of course the best way to do this, but since you're using jQuery for this I suspect that backward compatibility is a key issue here.