I use MindManager. It can link to some OF objects, but not all of them. (I think you can link to those that show up in Spotlight, but not the others?)
I find mindmapping useful for getting an overview of areas of focus and the like, but not so much in the day-to-day, task-by-task GTD process.
There's a two-page article about mindmapping and GTD in the second issue of Productive Magazine (
a pdf can be downloaded for free here) which might interest you. Didn't have too many practical tips, though.