for those who want to go a bit wider than 28mm and spend less, I've found
this page about using a Nikon .68x adapter, seems to be a good option, with 23.5mm FOV and not much loss of IQ.
1. Nikon WC-E68 .68x Conversion Lens (46mm thread) [edit: wow, it's like $150, I thought it'd be cheaper...]
2. Step-down ring : 49mm-46mm
3. 49mm UV filter (used as a spacer)
4. Fuji adapter (to get the 49mm thread)
In the post another adapter is mentioned, the Sony Fisheye converter for the NEX...
Anyone here have any direct experience with any of these? I'm considering getting this and giving it a try...