quick question: index optimisations on small tables

If I have:

CREATE TABLE small (
key integer PRIMARY KEY,
value text
);

and assuming there are only enough rows to fit in one page, doesn't it
make sense to use the index instead of a seq. scan for queries of type

SELECT value FROM small WHERE key = 12345;

Since it can get the answer straight out of the index, and if there are
potentially numerous rows, looking up a b-tree is faster than a linear
search?

- Andrew