How about talking to the guys behind Bubbl.us? That would be a cool integration too, as Bubbl is by far the best looking mind-mapping option out there.

I've done the copy and paste thing from Freemind and that does work.

However I am starting to prefer XMind and its text export option (one-click) inserts numbering as per http://screencast.com/t/MDUzOTBhNzk

Is there any way Checkvist can 'cut out' this numbering or would that be all up to XMind? (this is probably an obvious question).

freemind import vorks well (copy the freemind node and paste into the import text box)

.mmap would be great... especially since freemind can import mmap format

There's also Freemind and my paid favourite, Mindmeister.com

We were thinking of supporting export/import .mmap format. Is it what you need?