Database Schema with a shared identity Field
Until SQL Server supports sequences (next version "Denali") then you'll have to have a common table. 

However, if I understand you, I think you're looking at the subtype/supertype pattern. A sequence would be nice but if you designed using, say, Object Role Modelling then you'd generate this pattern/schema

Basically, you have a common "Contract" table:

ContractID, PK, UQ1 (Identity)
ContractType, FK, UQ1
InsertedDateTime
UpdatedDateTime
CounterParty


Notes:


the PK is ContractID to give a unique value
ContractType defines your "various types". Let's go for Tom, Dick and Harry as types
The unique key UQ1 is on both ContractID, ContractType (this is a "super key")
InsertedDateTime, CounterParty etc are common attributes


Then you have sub-tables with specific attributes for each contract type


ContractTom: ContractID, ContractType, TomAttrib1, TomAttrib2, ..., TomAttribn
ContractDick: ContractID, ContractType, DickAttrib1, DickAttrib2
ContractHarry: ContractID, ContractType, HarryAttrib1, HarryAttrib2, HarryAttrib3


Notes:


ContractID, ContractType are both the PK and FK.
ContractType has CHECK constraint to restrict to Tom or Dick or Harry