Why don't protons diffuse out of the mitochondria during chemiosmosis?
Diffusion, is by definition (Ficks Law) describing movement along or against some gradient (here its concentration) even if the mechanism of transport differ: active, passive, facilitated. The setup of the electrochemical gradient across membrane, like any thermodynamic process is not perfect, there are always entropy losses. In this case, this manifests as the the diffusion of protons away from the membrane (one source, there are others). Note the connection between entropy of system (Mitochondria) and diffusion (the inefficient process) in this positive feedback process. I think its interesting as someone learning about this stuff to make these connections.

Recalling protons are being delivered freely into the environment (inter membrane space) by Electron Transport Chain. Some protons will diffuse away from membrane surface before they can be pumped back in since these highly charged species cannot passively diffuse back in. The chemiosmosis process, which is sustained by the energy input to drive ATP synthase, depends on maintaining a strong concentration gradient (separation) of H+ on either side of membrane. Since the outer membrane of the mitochondria is porous, it means the protons are free to diffuse out, and will do so because it is entropically favorable. This relationship is best described by $\ce{\Delta G = RTln(K)}$. Where $\ce{K=\cfrac{[B]}{[A]}}$ for a process $\ce{A \rightleftharpoons  B}$ 

Where A= Protons near membrane, B= protons diffused out. Having established that thermodynamically they (protons) will diffuse out[3]. It begs the question will they actually reach the outer membrane in a reasonable time. 

Of course over long enough time they will. But over some meaningful scale like 1 second, the proton pump rate of ATP synthase, 100s-1 [5], how far can a proton get ideally?.
Back at the envelope calculation (ignoring any resistance effects from solvent, path, etc.) and using some approximate values (for yeast proteins and mitochondria):

From Fick's Rate Law, R, 
$ \sqrt[3]{R} = \sqrt[3]{ D A \cfrac{[H^+]_c - [H^+]_m}{d}} = 14 \mu m $

So in 1 s an average proton ideally can diffuse 14 micrometers from the inner membrane, driven by difference in pH between membrane and cytosol. I think i maybe be fair to presume that based on this the mitochondria is leaking protons into into its near surroundings constantly. (for scale the mitochondria is about 100nm).

Hope that was helpful

VALUES:

Thickness of mitochondria,$d$, equal 0.014 $\mu m $ [4]

$\ce{[H^+]_m }$ at membrane = $\ce{-log[H+] = 10^(-6.8),  [H+] = 10^(-6.8)}$ [6]

$\ce{[H^+]_c }$ at in cytosol = $\ce{[H+] = 10^(-7.40)}$ 

Diffusion coefficient of H+ in aqueous media $D$ = 7000 $\mu m^2$ [2]

A (surface area from which diffusion is taking place) I set to 0.025 $\mu m^2$ (guess from average unit cell of crystallized structure of ETC Complex II in yeast).

REFERENCES:


D. Nelson &amp; Cox (2012). Lehninger, Principles of Biochemistry, 6th Ed
https://esc.fnwi.uva.nl/thesis/centraal/files/f1163457446.pdf
http://www.ncbi.nlm.nih.gov/books/NBKhttp://www.atpsynthase.info/FAQ.html
http://www.ncbi.nlm.nih.gov/pubmed/9245766
http://www.atpsynthase.info/FAQ.html
http://jgp.rupress.org/content/139/6/415.full