Well this thread has managed to find three uses for the word "interleaving" in just two posts!
1) As I understand it, the buck regulators that supply the processors on PCs are run like a multiphase supply, so that they all produce current at different times. That will reduce the output ripple, the size of capacitors needed, and the size of the input current peaks.
2) With the very small number of turns on the transformer of an SMPS, there can be a real problem with stray inductance. That is the part of the inductance of one winding that is not linked to the other windings. The stray inductance can be reduced by winding half the primary, then the secondary, then the rest of the primary. It reduces the distance of the furthest part of the primary from the secondary.
3) Switch-mode supplies use ferrite cores and so don't have laminations, generally. The same applies to toroidal transformers. The rectangular transformers where the windings are wound onto a bobbin, and then the iron core is assembled usually have laminated cores. The cores have to be split so that they can be assembled around the bobbin. A common arrangement is EI where the lamination form a capital E and a capital I. If all the E laminations are stacked together, and so are all the I laminations, there is the possibility or a gap going the whole way through the transformer. Instead, the laminations are interleaved, and assembled EI on odd layers, then I3 on the even layers. **broken link removed**