My favorite mind mapping application is NovaMind. While I have not really done much in terms of working between iTHoughts HD and NovaMind, initial tests indicate they work together well. iThoughts does have a NovaMind export.
Here are iThoughts HD's pages addressing compatibility:
http://www.ithoughts.co.uk/iThoughtsHD/Compatibility.html
http://www.ithoughts.co.uk/iThoughtsHD/CompatibilityDetails.html
Their notes on NovaMind:
- When importing, iThoughts will recognise template colours and shapes (including the Rainbow scheme.) The template information is not re-exported.
- When exporting, iThoughts icons are embedded in the map - meaning that ALL icons are exported correctly from iThoughts. When importing, iThoughts can only map a subset of the Novamind icons.
MindManager seems slightly more compatible, but I do not the app nearly as much as NovaMind (although it does have a few features that NovaMind does not such as the ability to export an interactive PDF).