Today is message liberation day! Today's mandatory update, SL viewer 18.104.22.168 (which I'm installing now!) will contain a different way of handling the exchange of information between the SL viewer and the SL grid. Until now, the grid speaks only one version of the SL language, and the viewer has to match that version. Meaning, that whenever the SL grid language changes (almost every update), you have to download and install a new viewer, one that's compatible with the grid language version.
But after today's upgrade, that should change. The grid language and viewer language versions are, as of 1.18.x, no longer supposed to be 100% equal, all the time. When the grid language version changes, there will probably be an SL viewer update, but, according to Linden Labs, those upgrades may in many cases no longer be mandatory.
As IBM's Jaymin points out, this may boost open source development for the SL viewer. Until today, you could never be sure that what you developed for the SL viewer today, would still work with next weeks' mandatory upgrade. That may explain why open source development for SL has not taken off as much as Linden Labs probably have hoped when they open sourced the client. But after todays upgrade, your SL viewer will be valid for a much longer timespan, meaning that any development you do to this version, will also be valid and usable much longer. This makes development for the SL viewer a more sustainable and rewarding effort.
Somewhat related: Ajaxlife. If the grid's back online, be sure to try this one out. It allows you to enter Second Life through a browser. Not everything works, but basic movement and presence are there. It sounds very impressive.. but I have to try it out myself too, I discovered this tool while I was at work, and when I got home the grid was down :-)
For more information on Ajaxlife, and why this is so important, see this interview with Katharine on New World Notes: From the world to web.