</font><font color="blue" class="small">( It isn't vented because they want to keep moisture out of the unit. If it shot oil out when you opened the dipstick, then the unit must have been hot or overfilled. )</font>
Nope, it was cool and the level was correct. A couple of weeks after changing it, I removed the dipstick and got air pressure again. (Gee, maybe the oil is fermenting in there!) Also, when the dealer drained it to replace the seal the first time, he said it had quite a bit of pressure then too - that was before I vented it. They experienced the same thing on another unit as well.
I'm skeptical that proper venting would cause a significant moisture problem. I ran about 3 ft of fuel line up under the hood, ending with a lawn mower fuel filter. I doubt much air exchange will occur through that. Also, the transmission case appears to be vented, as are most automotive gear cases and axles I've seen.
By the way. the dealer recently mentioned getting a service bulletin from Kubota showing, in his words, their mickey mouse answer to the need for venting: a plastic vent plug that stands up a couple of inches above the top plug, ready to break off as soon as you hit some brush.