Inner join on two OR conditions dont use index

Hi all
i dont know if this is normal, but if yes i would like to know why and
how I could do it another way other than using unions.

(I tried on postgresql 7.4 and 8.0.3, made my vacuum analyse just before)
Here is my simple query:

select *
from rt_node n, rt_edge e
where node_id = 2
and e.start_node_id = n.node_id;

which give me the following query plan:

Nested Loop (cost=0.00..79.46 rows=24 width=60)
-> Index Scan using rt_node_pkey on rt_node n (cost=0.00..5.94
rows=1 width=36)
Index Cond: (node_id = 2)
-> Index Scan using rt_edge_start_node on rt_edge e
(cost=0.00..73.28 rows=24 width=24)
Index Cond: (start_node_id = 2)

But if I plug another condition with a OR like this:
select *
from rt_node n, rt_edge e
where node_id = 2
and (e.start_node_id = n.node_id or e.end_node_id = n.node_id);

I get this plan, it stop using the index!:

Nested Loop (cost=0.00..158.94 rows=4 width=60)
Join Filter: (("inner".start_node_id = "outer".node_id) OR
("inner".end_node_id = "outer".node_id))
-> Index Scan using rt_node_pkey on rt_node n (cost=0.00..5.94
rows=1 width=36)
Index Cond: (node_id = 2)
-> Seq Scan on rt_edge e (cost=0.00..81.60 rows=4760 width=24)

I tried SET enable_seqscan = OFF and it give me this (woah) :

Nested Loop (cost=100000000.00..100000158.94 rows=4 width=60)
Join Filter: (("inner".start_node_id = "outer".node_id) OR
("inner".end_node_id = "outer".node_id))
-> Index Scan using rt_node_pkey on rt_node n (cost=0.00..5.94
rows=1 width=36)
Index Cond: (node_id = 2)
-> Seq Scan on rt_edge e (cost=100000000.00..100000081.60
rows=4760 width=24)

These are my tables definitions:
CREATE TABLE rt_node (
node_id INTEGER PRIMARY KEY
);

CREATE TABLE rt_edge (
edge_id INTEGER PRIMARY KEY,
start_node_id INTEGER NOT NULL,
end_node_id INTEGER NOT NULL,
CONSTRAINT start_node_ref FOREIGN KEY (start_node_id) REFERENCES
rt_node(node_id),
CONSTRAINT end_node_ref FOREIGN KEY (end_node_id) REFERENCES
rt_node(node_id)
);

CREATE INDEX rt_edge_start_node ON rt_edge(start_node_id);
CREATE INDEX rt_edge_end_node ON rt_edge(end_node_id);

I cant figure why it cant use my index
I know I can use a UNION instead on two query like the first one only
different on "start_node_id"/"end_node_id", and it works,
but this is a part of a bigger query which is already ugly and I hate
using 5 lines for something I could in 5 words.

thank you!