I have been really trying to find a solid answer for this also,
I tried using the "search" key while in any of my numerous (including the new gmail account, that I pretty much was forced to set up), and whenever I press it and type in a term, it searches the internet, not my email....
I went to "searchable items" settings in options, and it only has apps, brower, contacts, corporate directory, kindle and music... no email.
This is a major function for me, as I have people asking me to forward them emails all the time, and it would take me forever to scroll through them to find them vs. with my blackberry, I would just click search, and be able to type in a variety of things to search by...