Terrible plan for join to nested union

I have a query which joins to a nested union and I'm getting a plan which never returns. Here is the query simplified as much as possible:

select 'anything' as result
from "Attribute" as A1
inner join
(
select R."TargetID" as "SourceID"
from "Relationship" as R
union
select A2."PersonID" as "SourceID"
from "Attribute" as A2
) as X on (A1."PersonID" = X."SourceID")
where (A1."ID" = 124791200)

(this seems like a strange query, but it is simplified to eliminate everything I could)

Here is the execution plan I am seeing:
http://explain.depesz.com/s/BwUd

Merge Join (cost=229235406.73..244862067.56 rows=727 width=0)
Output: 'anything'
Merge Cond: (r."TargetID" = a1."PersonID")
-> Unique (cost=229235336.51..233700093.63 rows=892951424 width=8)
Output: r."TargetID"
-> Sort (cost=229235336.51..231467715.07 rows=892951424 width=8)
Output: r."TargetID"
Sort Key: r."TargetID"
-> Append (cost=0.00..23230287.48 rows=892951424 width=8)
-> Seq Scan on public."Relationship" r (cost=0.00..5055084.88 rows=328137088 width=8)
Output: r."TargetID"
-> Seq Scan on public."Attribute" a2 (cost=0.00..9245688.36 rows=564814336 width=8)
Output: a2."PersonID"
-> Materialize (cost=70.22..70.23 rows=1 width=8)
Output: a1."PersonID"
-> Sort (cost=70.22..70.23 rows=1 width=8)
Output: a1."PersonID"
Sort Key: a1."PersonID"
-> Index Scan using "UIDX_Attribute_ID" on public."Attribute" a1 (cost=0.00..70.21 rows=1 width=8)
Output: a1."PersonID"
Index Cond: (a1."ID" = 124791200)

As you can see, the Relationship table has ~300 million rows and Attribute has ~500 million rows. I could not include the explain analyze because the query never completes. Going to "union all" fixes it, nesting the restriction fixes it, making the restriction limit X rather than A1 fixes it. Unfortunately, none of these "fixes" are acceptable within the context of the complete query this was simplified from.

Version string: PostgreSQL 9.1.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52), 64-bit
OS: CentOS 5
RAM: 128GB
Processor: AMD Opteron(tm) 6174, 24 cores

I've not changed any configuration settings from the based EnterpriseDB installer besides shared_buffers. Presently the DB is static, and I have executed analyze to update the stats since loading it.

Relevant schema:

CREATE TABLE "Attribute"
(
"ID" bigint NOT NULL,
"PersonID" bigint NOT NULL,
"Type" character varying(5) NOT NULL
)
WITH ( OIDS=FALSE);

CREATE INDEX "IDX_Attribute_PersonID_Type" ON "Attribute" USING btree
("PersonID" , "Type" COLLATE pg_catalog."default" );

CREATE UNIQUE INDEX "UIDX_Attribute_ID"
ON "Attribute" USING btree ("ID" );

CREATE TABLE "Relationship"
(
"ID" bigint NOT NULL,
"TargetID" bigint NOT NULL
) WITH ( OIDS=FALSE);

CREATE INDEX "IDX_Relationship_TargetID"
ON "Relationship" USING btree ("TargetID" );

CREATE UNIQUE INDEX "UIDX_Relationship_ID"
ON "Relationship" USING btree ("ID" );

Thanks,

-Nate