plpython returns integer[] fails for multi-dimensional array

In postgresql-9.0.1 I have to modify my plpython functions that return arrays.
It seems one dimesional arrays are handled properly, but not
2-dimensional arrays.

create or replace function atest() returns integer[] as $eopy$
a = list()
a.append(1)
a.append(2)
a.append(3)
#return a works fine
b = list()
b.append(a)
b.append(a)
# error
return b
$eopy$ language plpythonu

select atest() gives
obtest=# select atest();
ERROR: invalid input syntax for integer: "[1, 2, 3]"
CONTEXT: while creating return value
PL/Python function "atest"

How can I return multi-dimensional arrays in plpython?

TJ O'Donnell