hey james,
it really depends on what you want to achieve at the end.
for the bio itself: in your profile type settings (at "custom fields" tab) you could add a textbox or textarea field to your profile that would suit the input for a short bio. after this you can then re-arrange this field on the profile to place it in whatever position you want.

if you mean that you want to achieve the twitter look where this field value is then showing up on the cover photo area then that's currently not possible by default. however mark explained a manual custom workaround for this that may help you set this up here: http://stackideas.com/forums/is-there-a-way-to-add-a-field-other-than-gender-to-the-profile-homepage

hope that helps