Following example:

TableA:
[Column(Storage="_pkID", DbType="Int NOT NULL", IsPrimaryKey=true)]
public int pkID
[Column(Storage="_Groupname", DbType="NVarChar(50)")]
public string Groupname

TableB:
[Column(Storage="_pkID", DbType="Int NOT NULL", IsPrimaryKey=true)]   
public int pkID
[Column(Storage="_Name", DbType="NVarChar(50)")]
public string Name
[Column(Storage="_fkGoupID", DbType="Int")]
public System.Nullable<int> fkGroupID

This means, I have an entity of TableB which can be (but must not be)
assigned to a entity of TableA. Trying to set an association from
TableB.fkGroupID to TableA.pkID throws the errormessage. The Problem is, the
designer wants to declare an association between System.Nullable<int> and
System.Int - and this is just not the same datatype - the errormessage is
thrown.

For my usual work I have to deal with this. SQL-Server has no problems with
a join (inner or left) with this combination, joining "int" to "int NOT
NULL". Also Linq query ca do this:
var q = from b in db.TableB
join a in db.TableA on b.fkGroupID equals a.pkID //join int? to int
select XYZ;

Now my question is: How to deal with this?
I thought a big advantage of Linq to Sql was that I did not constantly have
to build joins by code - but with the designer it seems not to be possible...

-Stefan