Flies are an issue here too,some potted mint at the door is meant to deter them. That seemed to work last year.. But this year we are swamped with them.
I am planning on buying those hanging fly sticky paper things and put then outside the back door soon. I had considered buying a dog door, but Leo will just grow too quickly for it :/ (as I suspect so would Inca!)
There is a slot in pet door for sliding doors I have seen online
New! Ideal Fast Fit Fastfit Patio Cat Dog Pet Door NR - eBay (item 140386189792 end time Mar-03-10 18:32:30 PST)
Im considering making OH build something similar in a larger size.