Thinking about this a bit more, I suppose native support for MOBI files would be ideal (more supported formats = a more open device), but at least I can still buy books on Amazon if they're either
a) unavailable elsewhere or
b) cheapest on Amazon, and know that I can read them on my Glo.
If native MOBI support is a must-have feature, you'll need to look elsewhere. For my part, I'm not bothered about it not being there.