The sparse columns feature in SQL Server 2008 is generating lots of interest from people looking to deploy extensible schemas. I've seen a few questions from people that are confused by some of the info in Books Online, particularly about adding and removing sparse columns from a table.
There's a section in BOL that states that sparse columns are added and removed from existing tables by creating a new copy of each row *on the same page* and then deleting the old row, and that this can fail when the row size is around 4009 bytes (1/2 the max row size when sparse columns exist). I've been trying to repro this behavior as it seemed a nonsensical design to have used (and the design was done after I'd left Microsoft so I've never seen the underlying code) – and I couldn't. I finally got around to discussing this with the dev team last week and had it confirmed that Books Online is indeed incorrect – there is no such issue with sparse columns. I've been told that BOL will be corrected.
One other issue that's come up is whether sparse columns work with row overflow (i.e. rows greater than the size of a page, where one or more variable-length columns are pushed into off-row storage). The answer is yes, it works just the same as when the column isn't sparse.
Hope this helps some of you.