I've often found visibility manipulation would be useful alongside the usual manipulation of the display style via show()/hide()/toggle().

One proposal would be to create function reveal()/vanish()/toggleClass() to do the equivalent for visibility, but if that's too much new surface area for the API, how about a new option parameter, with a boolean "visibility" flag. So $("foo").show({visibility: true}).

Use case: A "loading" message shown below a form field. Toggling with show()/hide() causes the whole form to be displaced.

Note (1): I took a straw poll and found 8 out of 8 people want visibility in the core:

https://plus.google.com/106413090159067280619/posts/ZxRsZ9fyUtb

(Though I realise a poll like this is probably more biased towards people wanting new features.)

Note (2): As Paul Irish pointed out in the comments to that poll, there's an argument this should be about opacity rather than visibility, though that's probably something that could take place behind the scenes.