Hybridization molecule
This is a point of continuing drama.  Theoretical calculations as long ago as the 60s (cf. J. Am. Chem. Soc., 1994, 116 (10), pp 4414–4426) strongly suggest there is no d orbital participation in hybridized orbitals for Period 3 "hypervalent" compounds like the sulfite anion.  In fact, these calculations appear to suggest the observed bond shortening is probably explained quite simply by a significant ionic component to the bond, which means the best Lewis structures may well be the simple octet-rule following one, with 4 sp3 bonds around the central S, occupied by 3 bonding pairs (giving 3 sigma bonds) and 1 lone pair. No pi bond at all. This gives you higher formal charges than the resonance structure with a single double bond -- but that's what's meant by saying the calculations suggest the best VB description simply has more ionic content to the bonding between the S and O atoms (represented by the higher formal charges), rather than any d mixing.