Seems vBulletin only sets cache headers on Forum home:
Code:
<!-- no cache headers -->
<meta http-equiv="Pragma" content="no-cache" />
<meta http-equiv="Expires" content="-1" />
<meta http-equiv="Cache-Control" content="no-cache" />
<!-- end no cache headers -->
I've not noticed any issues on my own site using PocketIE on a windows mobile device, but it makes sense...