The current version of Windows has a hard limit on the number of logical processors it can support. For the sake of this discussion a computer has (CPUs X cores per CPU) logical processors; double that if using hyper-threading. This limit Windows actually supports is 32 logical processors on a 32-bit OS and 64 logical processors on a 64-bit OS. Most of these limits are subtle constraints based on data structures. For example, processor affinity, which determines which processors a thread can run on, is expressed as a 32- or 64-bit integer acting as a bitmask.
With SQL Server code named "Kilimanjaro" and Windows Server 2008 R2, the soft limit has been raised to 256 logical processors. The theoretical maximum is much higher, but Microsoft doesn’t want to support it until they have the ability to test it.