Currently, NANT offers only the ability to single task the processing of task elements, such as building files.
Visual Studio, unfortunately, also offers only the ability to utilize only one processor for building.
On a build machine with more CPUs available, there would be benefit to providing the ability to start building multiple project simultaneously.
And has a "parallel" element that offers this capability:
Note that suggestions of running multiple copies of nant misses the point. Starting multiple processes is not the hard part. Joining the independent "threads" together again is the key aspect, as well as queuing a maximum number of simultaneous workers (e.g. the "thread count" attribute of the parallel element in ant).