to_char() and order-by

Perhaps this was covered in the 7.2.x release notes, but it's mystifying us:

select to_char(time_link,'999.99') from stat_fetch
where time_link is not null
order by 1 desc limit 10;

(time_link is a double-precision column.)  Under 7.1.3, this returns results like:
to_char 
---------   9.99   9.99   9.99   9.99   9.99   9.99   9.99   9.99   9.99   9.99
(10 rows)

while under 7.2.1, you get the top 10 rows in *numerical* order:
to_char 
--------- 278.78 261.07 240.25 180.24 173.26 160.35 159.02 144.57 134.21 131.66
(10 rows)