When does Postgres cache query plans?

I'm curious under what circumstances Postgres will cache an execution
plan for a query.

Obviously if you create it with the PREPARE statement, it will be cached..

However, if I just run an ad-hoc query such as:

select * from Foo where X < 5;

A few hundred times, will that be cached?  What if I run:

select * from Foo where X < :value;

Can that be cached, or will it always be re-evaluated based on the
value of :value?  Thanks!

Mike