It’s that little no-no that we all (generalizing to make myself feel better) end up doing, adding user-scalable=no to viewport meta tags. Forms have a tenancy to cause iOS to zoom, it makes people (designers) a little crazy, and we opt out and add the meta tag to make the problem go away.
Recently, while working on the MailChimp redesign, we realized why the forms were zooming. Apple, with good reason, made a decision that when filling out forms on iOS they should appear at a comfortable reading size ~ 1em/16px. To accomplish this they zoom in on the page until that size requirement is met.
By setting our inputs to 1em/16px or higher, the OS no longer zooms and everyone is happy. Check out a little demo of differing input sizes to see it in action.
A little hack
It appears that there is one loophole around this if you really want your inputs smaller. By setting the font-size to >= 16px on focus we can keep fields the size we want until they’re in use. The font-size can’t have a transition applied however, so the scalling can feel a bit jumpy.