FYI. --Mike
A major update for LTspice was released today. LTspice IV, formerly known as LTspice/SwitcherCAD III, features multi-threaded solvers to better utilize current multi-core processors. Also included are new SPARSE matrix solvers that deploy self-authoring code which is assembled and linked on the fly in order to approach the theoretical flop limit of current FPU's. Large circuits run ~3 times faster on quad core processors. Small circuits will run at about the same speed as the prior version of LTspice.
Developing a parallel processing version of SPICE has been a long standing challenge in circuit simulation that has been met with limited commercial success. LTspice IV reflects a review of the techniques that have been attempted and implements proprietary methods that allow it to efficiently parallelize tasks that require as little as 5µs to run single-threaded in proportionally less time with additional processing cores.
LTspice IV requires a CPU at least as recent as the P4 and Win2K, XP, Vista, x64 variants or Linux. Support for earlier CPU's or Win95/98/ME is only through use of LTspice/SwitcherCAD III which is still available but is not expected to be further maintained.