From reading Ovid's post, I think there's a little misunderstanding about Juerd's post. Juerd is referring to the fact that PM's <img> tags for clear pixels have their "alt" attribute set to "clearpixel", which messes up text-only browsers as well as screen readers (this is an issue which pops up every now and then on the Evolt mailing list).
Specifying "" for the alt attribute generally works well, and the W3C seems to think so as well:
- Do not specify irrelevant alternate text when including images intended to format a page, for instance, alt="red ball" would be inappropriate for an image that adds a red ball for decorating a heading or paragraph. In such cases, the alternate text should be the empty string (""). Authors are in any case advised to avoid using images to format pages; style sheets should be used instead.
- Do not specify meaningless alternate text (e.g., "dummy text"). Not only will this frustrate users, it will slow down user agents that must convert text to speech or braille output.