Stuck in "group by" aggregate hell

I've got a table with the following schema:
Phone_num        latlon        location_when

Each row holds a user's phone number, their location, and the time they
were at that location.  There can be up to 120 rows per phone_num, each
with a  (potentially) different latlon, and each with a different
location_when (the primary key).

My problem - how do you get a list of each phone_num's most recent
position and time?  I want to weed out everything but a user's most
recent location, returning only one line per user.

Here's an example:

Phone_num            latlon            location_when
1111111111            22.12345,-90.12345    0901
1111111111            22.11111,-89.45678    0911
1111111111            21.99999,-89.55555    0921
2222222222            18.12334,-120.12345    1156
2222222222            18.10101,-120.11111    1206
2222222222            18.00001,-120.34889    1216


Given this, I want a list like this:
1111111111            21.99999,-89.55555    0921
2222222222            18.00001,-120.34889    1216



Obviously, it's something along these lines:
Select *,min(age(now(),location_when))
From table
Group by phone_num;

Unfortunately, Postgres wants me to group by latlon and location_when,
either of which makes each row a unique entity and causes me problems.

I'd prefer to not use temp tables, but at this point I'll take any
pointers I can get.  Intersect?  Some form of outer join with the same
table?

Thanks in advance!

Bret