Our Amie does this sometimes, pretty rare.
She kind of wretches a bit and we think she is going to throw up, but then she just stops and there is nothing. Once she kind of brought up what just look like saliva.
This has only happened maybe 3 or 4 times since we got her though.
Edit: Just found this
"Some dogs occasionally vomit a frothy, yellow bile a few minutes after waking in the morning, but this does not affect appetite and is not an indication of disease."
From what I remember Amie does this mostly in the morning after waking up. Is Penny the same?
Link to my flickr page with pics of Amie is in my profile, under "Contact Info"