1. We'll keep the main functionality free (with some reasonable limitations). We plan on adding some really cool collaboration and annotation/feedback features soon that we'll probably ask a buck or two for.
2. Yes, Mobile is one of our top priorities, simply because the mockups look so gorgeous on the Retina display and it makes sense. There are some challenges we need to overcome though (performance is one). We can at least promise some great additions on the Viewer veeeery soon.
