How do I check for nulls in an '==' operator overload without infinite recursion?

Use ReferenceEquals. From the MSDN forums:

public static bool operator ==(Foo foo1, Foo foo2) {
    if (ReferenceEquals(foo1, null)) return ReferenceEquals(foo2, null);
    if (ReferenceEquals(foo2, null)) return false;
    return foo1.field1 == foo2.field2;
}

Use ReferenceEquals:

Foo foo1 = null;
Foo foo2 = new Foo();
Assert.IsFalse(foo1 == foo2);

public static bool operator ==(Foo foo1, Foo foo2) {
    if (object.ReferenceEquals(null, foo1))
        return object.ReferenceEquals(null, foo2);
    return foo1.Equals(foo2);
}

Cast to object in the overload method:

public static bool operator ==(Foo foo1, Foo foo2) {
    if ((object) foo1 == null) return (object) foo2 == null;
    return foo1.Equals(foo2);
}