I don't think that anyone saw this coming as SAP has always preferred to "build" versus "buy". They have purchased some smaller companies like In-q-My (NetWeaver platform) and TopTier ("drag and relate" functionality), but have shied away from large acquisitions. This represents a major shift for SAP similarly when they began to "embrace" the Internet in the late 90's.
This move seems to be an admission by SAP that their core business is in decline (which Fortune 500 company isn't running one or more ERP systems?) and that NetWeaver isn't driving as much revenue as they hoped. The purchase seems to fit into their strategy of market accessiblity and its push into the mid-market. SAP has BI solutions for larger companies while Crystal Reports positioned for mid-market companies.
Apparently SAP is concerned about Oracle pushing into its BI territory with their acquisition of Hyperion. Oracle is has been on a buying spree for the past number of years (Siebel, PeopleSoft, Innobase, Sleepycat, etc.) and SAP is starting to see the need to broaden its portfolio.