SORT and Merge Join via Index

I noticed that when I have an index on (a,b) of table t, and I do an
SELECT * FROM t ORDER BY a ASC, it doesn't use the index.  When I
create a new index of only a, it does use the index.  Why is that?

And, more importantly, when I do a query involving a merge join of
table t, which requires sorting table t, the planner does the sort
manually using quicksort, not using the index. The time that step
takes is identical to the ORDER BY without using the index.  What do I
need to do to have Postgres use the index for the merge join?

(Postgres 8.3)

Thanks!