I've just read a good article which discusses using SQL Server in a hyper-threaded environment http://support.microsoft.com/kb/322385. To cut a long story short, we recommend that the maxdop (maximum degree of parallelism) setting should remain equal to the number of physical processors that are being used.
"...The performance of hyper-threaded environments varies. Conservative testing has shown 10 to 20 percent gains for SQL Server workloads, but the application patterns have a significant affect. You might find that some applications do not receive an increase in performance by taking advantage of hyper-threading. If the physical processors are already saturated, using logical processors can actually reduce the workload achieved.
For example, applications that cause high levels of contention can cause decreased performance in a hyper-threaded environment. We recommend that you test your application thoroughly to make sure that a hyper-threaded environment provides you the performance gain that you want versus the purchase of equivalent physical CPUs. Hyper-threading can be very helpful but hyper-threading cannot replace the full power of an additional physical CPU"