I had text originally for this hack too actually but liked the icon look for my site...images convey meaning to the brain faster than reading which is why I changed over to the icon.
If any user wants to substitute text for the icon just replace the <img> tag code with the text they want to see instead.
So if you prefer to use text instead of small "S" icon just replace the piece below from the original code with whatever text you want (i.e. subscribed):
PHP Code:
<img src=\"images/subscribed-icon.gif\" align=\"middle\" width=15 height=15 border=0 alt=\"You are subscribed to this thread. Click to view all subscriptions\">