Nice sum-up, Will.
justlukeyou, if this is your first book, there are a lot of side points to take into consideration. Likely your writing skills and even style will change a great deal as you progress, which could be awkward for the end-result of the trilogy. Often the first book just isn't as good as it needs to be in order to get published, but it teaches you so much. (I'm making generalizations, not addressing your personal skill.) However, if the first book isn't good enough to sell on its own, then the second book, no matter how awesome, is a part-two and can't even be offered on its own, which puts you in a bind. However, if you write book one and try to sell it and then start on an entirely different book, then if book one sells you can proceed with your series and you have another one started, maybe even completed. Whereas, if you finish book one and try to sell it and then start on book two, then you might end up with a trilogy of books which you can't sell because you never got enough interest in book one (for whatever reason). I think this is the base point most everyone has been making and what a few of us has experienced first hand. I had the first draft of book two written before it occurred to me that I had had absolutely no takers on book one and would be unable to have anyone look at book two without book one garnering interest. For me it was a wake-up call that I turned into a new personal rule: always write book one as a standalone (at least till you've been published) so that it doesn't end up trapping other books and never start on book two until interest has been shown in book one.
As a reader, I personally despise series books that end mid-story with no sense of finality--I feel cheated. However, something like what Once! delineated works fine for me because each book ends with a sense of tiered completion. Whether or not that's enough of a sense of completion for a publisher to take the book on without the others having already been written would likely depend on other details. No matter what, though, if the first book sucks then the first book sucks and I don't see why anyone would want to buy or read it, and if they did, why they would continue on through the series in the hopes that it improves.
As far as it being clear that you are writing a trilogy even with a standalone, you can always comment in your initial query that the book works as a standalone but is intended as the first in a trilogy. Personally, I have figured that the series potential of the book would be clear if the agent/publisher read it in its entirety and could be discussed after solid interest was expressed regarding the first book. No matter what, though, your first book has to be good on its own from the very start or it won't be selected by an agent or publisher.