Partial Index Too Literal?

Under somewhat unusual circumstances, rows in one of our tables have an 'active' flag with a true value.  We check for these relatively often since they represent cases that need special handling.  We've found through testing that having a partial index on that field works well.  What seems odd to me, however, is that the index gets used only if the query is a textual match for how the index was specified.

That is, using an index defined as 'where active = true':
dev=# explain analyze select * from result where active = true;
                                                             QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on result (cost=5.31..472.34 rows=4206 width=1126) (actual time=7.868..7.868 rows=0 loops=1)
   Filter: active
   ->  Bitmap Index Scan on result_active_idx  (cost=0.00..4.26 rows=2103 width=0) (actual time=4.138..4.138 rows=16625 loops=1)
         Index Cond: (active = true)
 Total runtime: 7.918 ms
(5 rows)

dev=# explain analyze select * from result where active is true;
                                                  QUERY PLAN
--------------------------------------------------------------------------------------------------------------
 Seq Scan on result (cost=0.00..537.26 rows=4263 width=1126) (actual time=55.631..55.631 rows=0 loops=1)
   Filter: (active IS TRUE)
 Total runtime: 55.668 ms
(3 rows)

This is version 8.2.6.  Is there something I'm missing that could make these queries ever produce different results?