since CS4 you’ve been able to rotate the preview of a page or spread, making it easy to review rotated content. you do it like this :
you’re not actually rotating the page, just the preview of the page. you’ll notice the pasteboard also gets rotated. a special icon in the pages panel shows you the preview has been rotated :
this is a fantastic feature for people working with packaging because artwork for boxes and whatnot usually have elements rotated 90, 180 and 270 degrees — all on the one page.
this is also really useful when you are working on a document to be produced as vertical spreads — like a wall calendar. traditionally these had to be compiled as single pages, but now an entire document can be set up in rotated spreads.
hopefully InDesign will someday allow for true vertical spreads because, brilliant as rotated previews are, there is one major drawback — it really makes it difficult when working out x and y ruler coordinates.