Last night at the NYC SQL Server Users Group meeting Linchi Shea presented a session on Data Compression (not Backup Compression) in SQL Server 2008.
By understanding how the compression algorithm works, and tuning your clustered indexes based on that understanding, you can achieve some pretty incredible space savings on disk, and performance benefits due to the increased amount of data loaded into cache. His analysis is available from his blog post SQL Server 2008 Data Compression: Impact of Data Distribution.
His explanation was clear and I learned a lot from his session. (I am in New York to do some performance testing at the Microsoft Technical Center, where the meetings are held every Fourth Thursday of the month.)
If you happen to be in New York and can spare the time, I highly recommend this group. It was well worth my time.