While I wouldn't disagree with Blake's basic points, I'd say the specific aspects of all this are as follows:
1. Sending out a CV/headshot to a casting director/agent is actually the accepted standard. Sending out a showreel is, in fact, uncommon.
2. Any unsolicited mail out is difficult to generate interest from. Blake's point is quite right, which is that there is no reason why anyone should question why you are sending a mail out - not only are agents and casting directors human beings, but fielding submissions is something that goes with the territory of their job. The real difficulty, and this I believe is what Forbes, Mark and Splat were referring to, is that most casting directors, in particular, take little interest in unsolicited applications - they are generally concerned with casting whatever project is currently on the table, and therefore looking for specific types, and it is a long shot, indeed, that your casting type will just so happen to meet the criteria for a part they are looking for the morning that it turns up. Almost always, interest from casting directors materialises from response to specific casting breakdowns.
Agents are a slightly different issue, because they are interested in you as a 'whole' actor, rather than a type, and so may genuinely be intrigued by your details sent on spec. But even agencies frequently feel you are too like someone already on their books, or consider that they cannot field any more clients, or are not interested in your particular skills base because they don't have expertise in areas you specialise in - it is really your job (if you want to save yourself time and money) to investigate which agencies are most suited to you before you send mails to them.
The other point that has been made, and is valid, is that it often useful to send any unsolicited mail asking for representation/audition or meeting, with a mention of either a current show you are performing in, an online source at which samples of your work can be viewed/link to a website, a mention of a broadcast programme/film that you are currently featured in and might be watchable etc. This at least gives the reader the option of knowing that there is material they can investigate further if they are interested in you as a prospect, and wish to get a better idea of whether or not you actually meet the grade as a performer. Very often, offers of this kind are waived - e.g. I have been offered interviews *without* anyone wishing to bother to come and see me in the show that served as initial impetus for the invite letter - but I suspect people appreciate the offer - at least, it gives them an indication that your work is out there.
3. Showreels are often not sent out in bulk mail outs. From your point of view, this is because they are expensive and not worth risking sending to those who will do nothing with them. If you are curious as to who/will not be prepared to accept showreel material, the best bet is either to make enquiries directly, or investigate a resource like 'The Actors Yearbook', which lists some of these details. From the point of view of casting directors, showreels are both desired and unwanted - on the one hand, those who cast screen work will be as interested to see showreel material as any other type of solicitation, as they can make casting decisions most effectively from it - but they may still have no time to view unsolicited showreels in the midst of their ongoing work, and don't always welcome the idea that the office is getting filled with showreels that they know they have little time to watch. For this reason, they offer ask that showreels *not* be sent unless requested. As I said to start with, the default is actually *not* to send a showreel with an unsolicited application, but to hold off unless asked to send one - the inclusion of 'showreel on request' in either covering letter or on CV is appropriate in these circumstances. If you prefer to make the rounds before sending anything, and ensure that sending a showreel to certain individuals is worthwhile, then you can do everything in the same mail out.
