Andy, as I understand it (and I don’t have much experience with screen readers, so I may be wrong here), the submenu items probably won’t be accessible as-is.
But at least on the sites I’ve worked on, dropdown menus are frills. They save you a step in your navigation, but if your top-level categories are intelligently designed, that’s all they get you—the top-level category you need is hopefully obvious, and when you follow the top-level link, the subnavigation will probably appear early on the index page anyway.
So I don’t know. I’m something of an accessibility (and standards) advocate, but I’m also a pragmatist. And I’d rather have pretty-good accessibility than none, particularly if the differential is small, and perfection is unattainable.
Of course, since I’m arguing that dropdowns are optional, it might be more equitable to have *no-one* get to have dropdowns, since after all, they aren’t really necessary.
I guess my question becomes “is the utility of dropdown menus for your audience such that it’s acceptable to offer them to only part of that audience?” And I can’t answer that for you. ...It might be the client’s call in a lot of cases, anyway.