Just had this one come to me from my team. It has been standard practice to embed any pre-text (or preview text if you would) into the alt tag of a 1x1 pixel gif in the head of all our campaigns. However, just recently, a client said that on IOS7, this trick isn't working anymore; and the preview text is instead pulling straight from the text version of the email instead.
Since IOS7 is such a big marketshare of email opens in general, I will discuss with our team that perhaps we should have a visible pre-header, as others in the industry have suggested and prescribed.
You can still hide pre-header text. I wrote a quick blog piece on how to do it which you can find here
Love to hear how you get on.
Will this work with Gmail? Last I checked, Gmail is still incredibly aggressive with forcing all display: none's into blocks.
Gmail still respects display: none !important. This is one of those rare cases where it comes in handy. It won't work in Outlook 07-13 however.
<div style="display: none !important; mso-hide: all">Preheader text</div>
Another technique is to make the preheader text the same colour as the background.