what are they planning in the way of importers/exporters to other formats?
As with most lower cost software, it would be usual to have some importers but few (if any) exporters.
Will it be sufficiently open/documented for people to write their own converters to/from their format?
I doubt it, in fact perhaps Altium are going to provide the low cost version for free but then charge you each time you want to generate gerbers (or native Altium files that can be used for production). This would be innovative (which Dave says it is) & would allow the hobbyist who makes only a couple of boards a year to use quality software at a low cost.
In fact, perhaps once you have finished your design, you log in to the Altium server, get your credit card out & Altium then converts the encrypted production file into gerbers/Altium production files. Maybe that cost will depend on the number of pads or the board size or perhaps the number of layers.
In fact the more I think about it (having read Dave's earlier hints), the more I think this is the way Altium is likely headed.
No worrying about 3rd party board shops paying to decrypt files & truly is a "user pay" service.
Encryption keys would be different for everyone making cracks much more difficult (a bit like PGP where 2 keys are required - one from you & one from Altium).