While waiting for MaxThink for Windows to solidify, I have also been looking into outliners for the PalmOS, as I have a new Visor. I came across Shadow Plan, which looks interesting, and which I am trying out at the moment but have not yet registered. I think it may become the first software that I actually purchase for my new toy, oops, tool.
The website says that the developer plans to support UserLand's OPML format.