>>173
メソッド構文ならSelectManyを使う
foreach (var v in list1.SelectMany(v1 => list2, (v1, v2) => v1 * v2))
{
Console.WriteLine(v);
}

この場合はクエリ構文の方が分かりやすいかも
var q = from v1 in list1
from v2 in list2
select v1 * v2;
foreach (var v in q)
{
Console.WriteLine(v);
}