window functions in an UPDATE

Is there a trick to using window functions to SET columns in an UPDATE?
Here is the query I'd like to run:

UPDATE profiles
SET score_tier = percent_rank()
OVER (PARTITION BY site_id ORDER BY score ASC)
WHERE score IS NOT NULL

But that gives me an error on Postgres 9.1:

ERROR: cannot use window function in UPDATE

This alternate version works, but is messier and slower:

UPDATE profiles p
SET score_tier = x.perc
FROM (SELECT id,
percent_rank() OVER (PARTITION BY site_id ORDER BY
score ASC) AS perc
FROM profiles p2
WHERE score IS NOT NULL) AS x
WHERE p.id = x.id
AND p.score IS NOT NULL

That second version is also prone to deadlocks if another job is updating
the profiles table at the same time, even with a query like this:

UPDATE "profiles" SET "updated_at" = '2012-11-13 21:53:23.840976' WHERE
"profiles"."id" = 219474

Is there any way to reformulate this query so it is cleaner, faster, and
not prone to deadlocks?

Thanks,
Paul

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Pulchritudo splendor veritatis.