Why is my index not used

I have a table, currently at about 150.000 rows, with a btree index on a
field named 'misscnt'. ~138k of the rows have this fields set as null, ~12k
as 0, ~350 as 1, ~200 as 2 and ~150 as 3 (these are the only values used).

Still, I get the following:

# explain select * from cam where misscnt>=1;
NOTICE: QUERY PLAN:

Seq Scan on cam (cost=0.00..3609.59 rows=46896 width=66)

Why do postgre think it will get over 46k rows from this query? If I try a
VACUUM ANALYZE on the table, I get:

# explain select * from cam where misscnt>=1;
NOTICE: QUERY PLAN:

Seq Scan on cam (cost=0.00..3874.01 rows=50347 width=66)

So now it actually thinks it will get _more_ rows:

# select count(*) from cam where misscnt>=1;
count
-------
692

If I use "set enable_seqscan=false;" and try "select * from cam where
misscnt>=1;" the index is used, and the query executes quite a bit faster.
However, shouldn't Postgre be able to do this automatically?

This is using PostgreSQL 7.1.2.

Thanks.

---
Kristian Eide