This would add an extra field to the custom profile field creation screen, with related extra database column. The field and db column would be of the same configuration as the description field - notably that the field is empty by default and is of type text. If no text is entered in the help field, the form is displayed exactly as currently.
If text is entered in the help field, this will add a help button to the field (wherever it is displayed), which when clicked displays the text entered in the help field as a normal Moodle help pop-up.
I have a number of requests for this (for our Sept 2011 release) and am more than happy to program myself and offer a patch. If whoever triages this thinks that's OK, please assign to me and I will add to the OU development schedule over the next few months.