First of all, what is Het-Grid?
Second Life wiki: "Het-Grid is short for Heterogenous Grid, a grid for which there is more than one version of server code running at the same time. Different versions of the server code may change which features are available on a particular region. Examples of server side features include how scripts are interpreted and run or how physics is simulated. This means that your viewer may play well with certain features, but not be able to access other features.
Viewer versions are different from server versions. The "viewer" is what we call the client application that you download onto your personal computer. The viewer is responsible for client side features such as drawing everything your avatar sees on your computer. The server is responsible for simulating how your avatar plays with other objects and avatars in that region.
The possibilities of having multiple server versions on the grid is important to residents of Second Life because depending on which region you go to, your experience may change. You will receive a notification when you enter a region with a different version than the region you came from so that you can judge for yourself whether or not you want to stay in that region. Beta regions are running versions that are being tested. If you enter a beta region, you may experience a significant difference in performance, so it is important to read release notes on beta regions before doing anything like inventory access.
Server versions and viewer versions are both grouped by common functionality. We call these groupings channels. Examples of server channels are "Second Life Server" which contains general improvements to the main Second Life server code and "Mono Beta Server" which contains a set of beta versions that make improvements to a new Mono script execution system. Examples of viewer channels are "Second Life Viewer" which contains general improvements to the main Second Life viewer code and "Firstlook" which contains a set of versions that provide new features that are in development. The first new feature to appear in the First Look channel was 3D Voice."
So far the explanation of the Het-Grid.
According Second opinion's newsletter, almost all future viewer updates for Second Life will be optional, so you can upgrade whenever you want.
Het-Grid will provide Second Life a smoother transition between both small and major patch releases. Linden Lab understands that by having separate beta grids, we aren't able to fully test new versions of Second Life viewers and servers. With Het-Grid, we will be able to release new server versions to small areas of the Main Grid for testing. This allows Residents to beta test new server upgrades without having to disconnect from the primary Second Life grid and empowers Linden Lab to update the Second Life servers without creating grid-wide downtime. So, viewer and server versions will be forward and backward compatible.With the ability to keep the same viewer versions when you want to and with your help on the new beta servers, Linden Lab hopes to give everybody a much smoother experience in Second Life.
Let's hope this will stabilize the grid, because no one is waiting for another downtime as we had yesterday evening, for the Europeans that is.