Why is it choosing a different plan?

Hi all, I'm having some confusion with the 7.4 query planner.

I have two identical queries, whereby the passed (varchar) parameter
appears to be the deciding factor between a sequential or an index scan.

IE, This query:

explain SELECT DISTINCT (a1.ENTRY_ID) AS retrieved FROM OS_CURRENTSTEP
AS a1 , OS_CURRENTSTEP AS a2 WHERE a1.ENTRY_ID = a1.ENTRY_ID AND
a1.ENTRY_ID = a2.ENTRY_ID AND ( a1.OWNER = 'p1' AND a2.STEP_ID =
1 );
NOTICE: QUERY PLAN:

Unique (cost=1175.88..1175.88 rows=1 width=16)
-> Sort (cost=1175.88..1175.88 rows=1 width=16)
-> Nested Loop (cost=0.00..1175.87 rows=1 width=16)
-> Index Scan using idx_9 on os_currentstep a1
(cost=0.00..1172.45 rows=1 width=8)
-> Index Scan using idx_8 on os_currentstep a2
(cost=0.00..3.41 rows=1 width=8)

However, this query:

explain SELECT DISTINCT (a1.ENTRY_ID) AS retrieved FROM OS_CURRENTSTEP
AS a1 , OS_CURRENTSTEP AS a2 WHERE a1.ENTRY_ID = a1.ENTRY_ID AND
a1.ENTRY_ID = a2.ENTRY_ID AND ( a1.OWNER = 'GIL' AND a2.STEP_ID =
1 );
NOTICE: QUERY PLAN:

Unique (cost=3110.22..3110.22 rows=1 width=16)
-> Sort (cost=3110.22..3110.22 rows=1 width=16)
-> Nested Loop (cost=0.00..3110.21 rows=1 width=16)
-> Seq Scan on os_currentstep a1 (cost=0.00..3106.78
rows=1 width=8)
-> Index Scan using idx_8 on os_currentstep a2
(cost=0.00..3.41 rows=1 width=8)

Thoughts about why changing OWNER from 'p1' to 'GIL' would go from an
Index Scan to a Sequential?

[There is an index on os_currentstep, and it was vacuum analyze'd
recently.]

Running version 7.4 (working on upgrading to 8.0 soon). Thanks!

--
Anthony