Lindens Labs has announced a rolling upgrade, where they restart all of the sims in order starting with the northwest corner of the grid and going north to south. You will get, or should get a five minute warning, many times I have received no warning, I assume due to lag.

This will once again be a traditional rolling upgrade, starting at the northwest corner of the world and moving from north to south in a big wave. Residents will get a 5 minute warning and can teleport to another region. Source: Rolling Upgrade on Wednesday Oct. 10th, 9am PDT (16:00 GMT)

In an update to the blog post from Second Life, the Rolling Restart is complete.

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Second Life will be down next Wednesday, August 22, 2007 from 9:00am until 11:00am SLT, the Lindens need to complete their deployment of the code they started last week. They expect it to take a couple hours, but it could go longer. They will allow logins but will use a rolling restart to allow the region’s to pick up the new code, and that should take about three hours. This will introduce the Het Grid to the system that will allow the Lindens better control over grid updates.

From one of the first announcements concerning the Het Grid,

Very soon, Linden Lab will release Second Life version 1.18.0. As Zero posted about “Message Liberation,” almost all viewer updates will be optional, so you can download them if and when you want. Message Liberation lays the groundwork for a project known as “Heterogeneous Grid,” or “Het-Grid” for short. Het-Grid will provide Second Life a smoother transition between both small and major patch releases. We have understood for a while now that by having separate beta grids, we aren’t able to fully test new releases thoroughly. With Het-Grid, we will be able to release new server versions to small areas of the Main Grid for testing. This allows Residents on the Main Grid to beta test without having to connect to a separate grid. Source: