Why are distinct and group by choosing different plans?

I notice that I get different plans when I run the
following two queries that I thought would be
identical.

select distinct test_col from mytable;
select test_col from mytable group by test_col;

Any reason why it favors one in one case but not the other?

d=# explain analyze select distinct test_col from mytable;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------
Unique (cost=0.00..14927.69 rows=27731 width=4) (actual time=0.144..915.214 rows=208701 loops=1)
-> Index Scan using "mytable(test_col)" on mytable (cost=0.00..14160.38 rows=306925 width=4) (actual time=0.140..575.580 rows=306925 loops=1)
Total runtime: 1013.657 ms
(3 rows)

d=# explain analyze select test_col from mytable group by test_col;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=7241.56..7518.87 rows=27731 width=4) (actual time=609.058..745.295 rows=208701 loops=1)
-> Seq Scan on mytable (cost=0.00..6474.25 rows=306925 width=4) (actual time=0.063..280.000 rows=306925 loops=1)
Total runtime: 840.321 ms
(3 rows)

d=# select version();
version
----------------------------------------------------------------------------------------------------------------
PostgreSQL 8.2.1 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)
(1 row)