'_' < '5' -- different answer on 7.2 and 7.3

I noticed a change between our 7.2 and 7.3 postgresql database.

On 7.2:

template1=>  select '_' < '5';
  ?column?
----------
  f
(1 row)



On 7.3:

template1=#  select '_' < '5';
  ?column?
----------
  t
(1 row)


Any reason for this change?

dave