PostgreSQL 8.3: Slow GROUP BY on large table
So why does Postgres 9.2 still show a sequential scan? I quote the Postgres Wiki:


  Is "count(*)" much faster now?
  
  A traditional complaint made of PostgreSQL, generally when comparing
  it unfavourably with MySQL (at least when using the MyIsam storage
  engine, which doesn't use MVCC) has been "count(*) is slow".
  Index-only scans can be used to satisfy these queries without there
  being any predicate to limit the number of rows returned, and without
  forcing an index to be used by specifying that the tuples should be
  ordered by an indexed column. However, in practice that isn't
  particularly likely.
  
  It is important to realise that the planner is concerned with
  minimising the total cost of the query. With databases, the cost of
  I/O typically dominates. For that reason, "count(*) without any
  predicate" queries will only use an index-only scan if the index is
  significantly smaller than its table. This typically only happens when
  the table's row width is much wider than some indexes'.


Emphasis mine.
There is hardly anything to gain from an index scan here, as long as your talbe isn't bloated with more (big) columns.