My understanding (based on information through work) is that the higher risk areas are to the north of the country and in particualr around the border region with SA. Generally, the kidnap risk in Yemen is related to tribal differences and in almost all cases the tribes use the "victim" as a bargaining chip for their objectives - which are not really financially related. In almost all cases the victim is released un harmed. A friend of mine has been "kidnapped" three times in the same region because the locals wanted a road sealed.
The recent cases are a departure from the norm in Yemen and are certainly cause for concern, but there is no evidence that this is a new development in Yemen per se, some suggest that the reason the victims were killed was due to a botched recuse attemtp but I don't know about that.
Best to check the FCO guidelines on the area but I think if you stay to the south you should be OK.