Wow - I was expecting more folks to have talking dogs....
Regarding teaching Ben: he was the one who first said "owf", and I thought it was funny, but of course jumped up to let him out. So I sort of reinforced the "word". Later, he'd try to get my attention by saying "arf" or "bark" - and I'd say "say owf" and wait a bit... the idea being that the right sound gets immediate attention, and the wrong word gets a small delay. He's a 2-yr-old; I'm not doing this with a puppy. I'm also not expecting him to say a sound he doesn't already "say".
My dogs don't do "tricks" (they DO know basic obedience, of course) - but having them able to communicate either with sounds; or simply by "showing" me what they want is pretty darn handy. More than once my dogs have alerted me to boats floating away because they became untied; or a herd of geese in the yard, etc. They are SOOOO much more observant than we are :-)