PostgreSQL: COUNT(*) uses a sequential scan, not index
The official wiki pages give an answer to that:


  [...] The reason why this is slow is related
  to the MVCC implementation in
  PostgreSQL. The fact that multiple
  transactions can see different states
  of the data means that there can be no
  straightforward way for "COUNT(*)" to
  summarize data across the whole table;
  PostgreSQL must walk through all rows,
  in some sense. This normally results
  in a sequential scan reading
  information about every row in the
  table. [...] 


Furthermore, you could try an ANALYZE to rebuild the info for the query planer. 

You should get a better performance using COUNT(an uniquly indexed field) but if this is very big, a seq scan is the only way to do it.

If you need very quick numbers and are not afraid of querying the schema, you can do the following

SELECT reltuples FROM pg_class WHERE oid = 'your_table'::regclass


But don't rely on this values as it is only an "estimated" (although often the exact) number of tuples in the table.