var ids = new List<int> { 6, 8, 9, 10 };
var query = from x in ctx.SomeTable
            where ids.Contains(x.Id)
            select x;

------解决方案--------------------

var listOfIds= new int[]{5,6,7,8};
var context=CreateEntityFrameworkObjectContext();
var results = from item in context.Items
             where listOfIds.Contains(item.Category.Id)
             select item;

------解决方案--------------------

大致原理如下，逆向实现。
C# code
int[] idset = new int[] { 5, 6, 7, 8 };
var result = mydatarows.Where(r=>idset.Contains(r.id));

------解决方案--------------------

var listOfIds= new string[]{"5","6","7","8"};
var context=CreateEntityFrameworkObjectContext();
var results = from item in context.Items
 where listOfIds.Contains(item.Category.Id.ToString())
 select item;

------解决方案--------------------

实测结果如下，尽管LINQ to Entity与LINQ to SQL Entity有一些差异，在有比较子的运算符上可能需要自己实现，不过你这里的需求应该是可以直接实现的。
C# code
using (NorthwindEntities db = new NorthwindEntities())
{
    string[] cities = new string[] { "London", "Paris", "Berlin" };
    var result = db.Customers.Where(r => cities.Contains(r.City));

    foreach (var r in result)
    {
        Console.WriteLine(r.ContactName);
    }
}