Indexing a boolean

I would appreciate it if I could get some thoughts on indexing a field
with only two values?  For example, I have a table with a few million
rows in it.  All items in this table are broken up into two categories
using 'T' or 'F'.  It seems logical to me that an index on this field
would create two logical 'buckets', so that one could say, "I want all
'T' values", or "I want all 'F' values" and merely have to look in the
appropriate bucket, rather than have to perform a sequential scan
through three million items every time a request is made based on 'T' or
'F'.  If I were to create an index on a char(1) field that contains only
values of 'T' or 'F', would the query analyzer override the use of this
index?  How does Postgres address this problem and what are all of your
thoughts on this issue?  I appreciate the help!

Kris