__attribute__((__aligned__(16))) is applied to an empty base class, used just to propagate that and new/delete operators, not defined in the example (do you remember SSE...).
If derived class T is smaller than the alignment, it is obeyed so arrays of T give aligned alements. If size of T is bigger that alignment, elements in T[] are unaligned. If base is not empty, it works ok. Is this expected behaviour ?

Release:
gcc-3.2.2

Environment:
Linux Mandrake 9.1

How-To-Repeat:
#include <iostream>

using namespace std;
#define __class class __attribute__((__aligned__(16)))

__class A
{
public:
    void id() { cout << this << endl; };
};

class T : public A
{
public:
//    with 3 floats, sizeof(T)==16
//    with 5 floats, sizeof(T)==20
      float   f[5];
};

int main()
{
    A a;
    cout << "sizeof(A) = " << sizeof(A) << endl;
    cout << &a << endl;
    A aa[2];
    cout << "sizeof(A[2]) = " << sizeof(aa) << endl;
    cout << &aa[0] << endl;
    cout << &aa[1] << endl;

    T t;
    cout << "sizeof(T) = " << sizeof(T) << endl;
    cout << &t << endl;
    T tt[2];
    cout << "sizeof(T[2]) = " << sizeof(tt) << endl;
    cout << &tt[0] << endl;
    cout << &tt[1] << endl;

    return 0;
}