Hi Ray,
"Placeholders" are text that appear, typically in a light grey color, _inside_ an entry box when the entry box is empty which give the user some idea of what to type. For example in date fields it's common to put the date format (mm/dd/yyyy) (*) or in locator fields to put "search" - stuff like that.
(*) The date thing is actually a good example, but actually the user can type in any format they like into a date field, and it'll figure it out. if you enter 31/12/2010 it knows that is dd/mm/yyyy even if the format asked for is mm/dd/yy. You can even type in stuff like "tomorrow" or "next week" or "3rd wednesday in june" and it's still fine.
HTML5 is mostly a natural evolution of HTML - there's nothing magical about it - but yes it has a bunch of new features. Of course we already make use of many of those features, and I'll continue to adopt others as time permits. So far all the features used are backward-compatible so actually we're generating HTML5 already - but there's certainly more in HTML we can use - the pace is really dependent on the pace that the browser makers implement stuff on their side.
cheers
Bruce