I don't think there are rules explicitly for this, but I have a suggestion.
Compare Earth Glide (Ex) to Incorporeal (Ex) and borrow some of the "pass through solid objects" restrictions.
- Don't need to remain adjacent to the object's exterior — Earth Glide is clear on this point.
- Can sense creatures directly adjacent.
- In order to see beyond the object it is in and attack normally, the creature must emerge.
Unless the druid has other senses, Earth Glide doesn't let him sense any more than he normally would and being embedded in a wall is somewhat limiting. Likewise, he doesn't have any line of effect to anything (other than the wall or himself) while in the wall.