To my knowledge,
db_select() isn't aware of entities or fields (fields, in the context of field that are added to nodes etc.)
I imagine you'd need to do the joins to field tables yourself. I found a little example from dbtng_example--dbtng_example.module, but as per my hunch and comments below -
EntityFieldQuery is actually the only correct way to do this. Fields can define their own storage which would completely break your db_select, so...
With a bit of a performance hit (which is negligible for most) you really should do it the Drupal 7 way...
Drupal 7 now has Fields in core, and with that came the EntityFieldQuery which is a querying class for finding entities (like nodes, terms, etc) by their properties (like published status) or fields (like field_page_photo or any other fields you add via UI or otherwise)
or from API:
This class allows finding entities
based on entity properties (for
example, node->changed), field values,
and generic entity meta data (bundle,
entity type, entity id, and revision
Some good examples and discussion here:
What's the proper use of EntityFieldQuery?
and I find myself referencing Do we need an EntityFieldQuery example? as well.