If you only want other people to be able to read your document, PDF is the most sensible option: it more or less guarantees a perfect reproduction of your document's formatting without placing a burden on the reader to use the software you used to create it. If you want other people to be able to edit your document, it only makes sense for you to come to a common agreement as to which editor you want to use (GNOME Office, OpenOffice, Microsoft Office, whatever). If you don't, formatting information will (almost always) be lost and people will be annoyed.
Unless and until XHTML supports a strict superset of the formatting options of all word processing applications out there (which seems rather unlikely), I don't see it having a large tangible effect. Actually, the only thing that I see changing the above set of recommendations is the outright displacement of office software (in the mold of Microsoft Office) from the software landscape.