A number of factors can affect CPU utilization on a database server, including compilation and recompilation of SQL statements, multithreaded operations, disk and memory bottlenecks, and ETL activity, among others. You'll want to make sure that the CPU is spending its time processing what you want it to process and not wasting cycles on poorly optimized code or sluggish hardware.
In the October 2007 issue of TechNet Magazine, Zach Nichter explains how you can optimize your hardware and your queries for better SQL Server CPU utilization.
Zach also takes a look at Hyper-threading, and whether it helps or hinders performance on your database server.
Plus, you'll find lots more articles, downloads, virtual labs, and webcasts on SQL Server at the SQL Server TechCenter.