I did a little asking around the office and found that 1.3 Vso as a single standard value is no longer used, in transport catagory airplanes at least. The value is variable can be down to the low 1.2s (the CRJ900's Vref is 1.23 Vso) and is a function of various factors that takes into account stall behaviour, the spread between Vref and V2ga etc etc. Basically Vref is set to be as slow as possible while having sufficient energy for stall margin, arresting sink at the flare, and the transition from Vref to climb at V2 - V2+10 for the go-around case.