Rob, Farrah,
Unfortunately you can't set visibility per field as this list is generated in code. I'll make this available in the next major upgrade. Having said that, here is a general note about changing pages.
There are 2 routes to modify the so-called ViewDetails page: you can edit the ViewDetails.ascx or you can create a new one. The second option allows you to create a different ViewDetails for every portal if that is necessary as you can change the 'wiring' by specify in the DMX_Extensions table what ascx to load for ViewDetails. The first will just alter it for everyone.
When altering the page keep in mind that as a general rule you must not remove any elements that have runat="server" in the tag. Having said that, you can easily edit the html to make stuff disappear. you can specify on most regular html elements style="display:none" and it will disappear. For asp.net elements you can add Visible="False" (note that this is not 100% guaranteed as the visiblity might be set in the code). Finally you can enclose parts of a page using div style="display:none" to remove stuff.
General precaution: keep a copy of the original ascx when you start to edit and do it progressively, testing every step along the way.
Peter