I'm intrigued by:

catch (Exception e)
    {
        Error = e;
    }

?
>I don't know what the hell I was thinking. What I wanted was:
>
>

>using (AppSecurityDataContext dc = GetDataContext())
>{
>    tblUsersInApplication uiaRow = (from uia in dc.tblUsersInApplications
>                                    where uia.UserId == um.UserId &&
>                                            uia.ApplicationId == LoginCredentials.ApplicationId
>                                    select uia).FirstOrDefault();
>
>    uiaRow.LastLoginDateTime = DateTime.Now;
>
>    try
>    {
>        dc.SubmitChanges();
>    }
>    catch (Exception e)
>    {
>        Error = e;
>    }
>}
>

>
>Now it works. I gotta quit drinkinig when I'm coding...
>
>
>
>>>>>Why doesn't this work????
>>>>>
>>>>>

>>>>>using (AppSecurityDataContext dc = GetDataContext())
>>>>>{
>>>>>    tblUsersInApplication uia = new tblUsersInApplication
>>>>>    {
>>>>>        UserId = um.UserId,
>>>>>        ApplicationId = LoginCredentials.ApplicationId,
>>>>>        LastLoginDateTime = DateTime.Now
>>>>>    };
>>>>>
>>>>>    try
>>>>>    {
>>>>>        dc.SubmitChanges();
>>>>>    }
>>>>>    catch (Exception e)
>>>>>    {
>>>>>        Error = e;
>>>>>    }
>>>>>}   
>>>>>

>>>>>
>>>>>I get no exceptions. Runs fine. The date/time doesn't show up in the SQL table after it runs.
>>>>
>>>>Is this Entity Framework? (I'm guessing it is.)
>>>>
>>>>You need to add the new item to the context, if it's not a child entity EF will have created a function something like this:
>>>>
>>>>dc.AddTotblUsersInApplication(uia);
>>>>
>>>>If it's a child you just add it to the reference collection on the parent entity.
>>>
>>>No, it's just a plain old Linq statement. Not EF.
>>>
>>>And, I have a bunch of other queries just like this one that work fine.
>>
>>"By plain old Linq" do you mean Linq to Sql?
>>
>>If that's the case, Michael Collins gave you the correct syntax:
>>

dc.tblUsersInApplications.InsertOnSubmit(uia);

>>
>>or if you have a child relation by adding it to the child collection on the parent object.
>>
>>The DC has to know about the object in order for it to be persisted back to the store.