My advice that, create 2 new layer called DTO and Domain Objects

1. For UI layer (it can be web application , windows application). This will call BLL
2. For Transfer data, - DTO/Business Objects between UI and SOA (it makes as the datacontract - proxy)
3. For BLL - we are planning to implement WCF. This will call DAL
4. For DAL - we are planning to implement class library project which will call stored procedure for database(which can be sql server/oracle)
5. For Transfer data, Domain objects,just like result set of database/tables; between DAL and BLL

Here the data flows goes like

DAL <--> DO <--> BLL/SOA <--> DTO <--> UI

Also as wikkard said, you can implement dependancy injection framework.

All the best!!!