'_' < '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