Bitmap indexes

I was wondering about index types. Oracle has an index type called a
'bitmap' index. They describe this as an index for low cardinality
fields, where only the cardinal values are indexed in a b-tree, and
then it uses a bitmap below that to describe rows. They say that this
type of index is very fast when combined with queries that used the
indexed row in 'AND' clauses in a sql statement as the index can
'mask' the results very fast. I have not been able to benchmark the
actual effectiveness of this kind of index, but I was wondering if
anyone has had experience with this an believes it might be a useful
feature for postgres?

Yes I have a vested interest in this because alot of my searches are
masked against low cardinality fields 'Y' or 'N' type things where
this could potentialy benefit me...

Alex Turner
NetEconomist