I'm like Russ, i don't really care about the cheating, only about Summer; it happens Summer' story arc is about Owen/Cristina relationship, that's all.
More, it allows me to create a new poll inside the forum
As you Alex and Russ guessed, Summer may not be involved on the cheating. An inside source says Owen has been seen in a bar at night with a young woman (it remains to be confirmed but her description does not fit Summer Glau), which means the cheating might be real but not necessarily with nurse Emily.
That said, i doubt GA's writers will focus that much on nurse Emily, especially if she has not an affair with Owen; they want to emphasize the problems between Cristina and Owen (see their visit to a marriage conseillor you mentioned previously).
Shonda Rhimes words on twitter :
this (i.e. Cristina's fear that Owen is cheating on her) is gonna be PAINFUL and THEN there is gonna be growth.