It appears there is a bug in Internet Explorer (tested on IE8).
When selecting Import Leads / Import Organization Leads, once the file is chosen and "next" is selected, it automatically switches to "Import My Leads" on the next page, and the user cannot select a column in the spreadsheet to specify who the lead owner should be.
This does not happen in Chrome or Firefox, only IE (but is of little comfort to someone who is forced to use IE at work).
Thanks