I just found what I think is going to be my default text editor for the next couple of years: Tomboy. Tomboy is a neat Gnome application that does a couple of things better than the typical notepad.
The most distinguishing feature is a wiki-like system which allows you to create links on the fly and creates new notes out of them. This is great for taking notes in index-card fashion.
Then there's the search facility which allows you to sift through the contents of your notes.
It's also integrated with the Gnome desktop so you can call it with just a custom keypress combination.
There's a built-in spellchecker which I don't really like having turned on by default. There are supposed to be other plugins but I haven't seen a whole lot of them yet. And one quirk is the lack of a proper printing facility as of the moment, but that doesn't bother me because I don't print anymore.
Files are in XML form so they should be easy enough to convert.
The whole thing is written in C# and uses the Mono runtime, by the way.
Give it a spin and see if you like it.