Read the Copyright Act, sections 30(4), (5) & (6):
(4) Where a literary, dramatic or artistic work is made by the author in pursuance of the terms of his employment by the proprietor of a newspaper, magazine or similar periodical under a contract of service or apprenticeship and is so made for the purpose of publication in a newspaper, magazine or similar periodical, the proprietor shall be entitled to any copyright subsisting in the work by virtue of this Part insofar as the copyright relates to
(a) publication of the work in any newspaper, magazine or similar periodical; or
(b) reproduction of the work for the purpose of its being so published,
but not otherwise.
(5) Subject to subsection (4), where
(a) a person makes, for valuable consideration, an agreement with another person for the taking of a photograph, the painting or drawing of a portrait or the making of an engraving by the other person; and
(b) the work is made in pursuance of the agreement,
the first-mentioned person shall be entitled to any copyright subsisting in the work by virtue of this Part, except that if the work is required for any particular purpose, that purpose shall be communicated to that other person and that other person shall be entitled to restrain the doing, otherwise than for that particular purpose, of any act comprised in the copyright in the work.
(6) Where a literary, dramatic or artistic work to which subsections (4) and (5) do not apply, or a musical work, is made by the author in pursuance of the terms of his employment by another person under a contract of service or apprenticeship, that other person shall be entitled to any copyright subsisting in the work by virtue of this Part.
What all that really means : is that, under section 30(4), if you're given valuable consideration, ie. being paid, either through regular employment or on a per-contract basis, to take photographs for any person or agency for publication, the rights to the photograph belong to THEM, not to you, unless you're using the photographs outside of the contracted parameters. In all other instances of shooting under employment, you have NO copyright, unless you've specifically contracted for it. It doesn't make any difference whether you're employed or a free-lancer.
Practical advice: if you're shooting for an agency or a magazine, don't use the photos for any other commerical purpose, for frankly, the only way to use photographs is to publish it somewhere, isn't it? In any case, inform the employer, get a release or consent. Or specify your terms of use in your contract.
Hope this clears it all up for those of you pros, there's certainly been lots of queries on the topic. Feel free to chip in with more opinions, y'all.