There's now a 3rd party web based SL client called AjexLife in development and available. So far it has more or less the same feature set as Slink, meaning it's good for logging into the grid, chatting, sending instant messages, getting notifications, viewing the map and teleporting around. Of course - no graphics. The main focus is on for the main client underpowered machines and communicating so far.
Since this works with AJAX, it should be possible that it even works through most company firewalls - I guess the server makes the connection to the grid, not the webbrowser per se. I wonder under which license this work is going to be put under...