Can I vent a bathroom through the soffit if I use a product designed for it?
In the past I was a big fan of soffit vents. I figured if you used quality vents with a good back-draft damper, and sealed and insulated the duct well, it was a great way to vent bathrooms. There are several key benefits:


First and foremost, it avoids penetrating either the roof or a gable 
wall, so it eliminates any risk of liquid water intrusion from 
wind-blown rain or unsheltered gable walls. However problematic humid
air is for an attic, I think liquid water is an order of magnitude
worse..
It also allows for short efficient duct runs, keeps the duct low (in between
the floor joists), and therefore well underneath blown in insulation
(so you can use rigid duct and avoid the need for separate
insulation).
Finally using a soffit vent sort of creates a thermal trap- warm air
from inside the duct wants to rise not sink, so there should be less
heat less when the exhaust fan isn't running. Even with dampers at
the fan and the vent, those dampers are certainly not airtight. Often
the end of the duct often goes up a little bit before going down and
out the soffit vent, so that only enhances this thermal trap.


However despite all these benefits, which I think are significant, I'm starting to reconsider whether soffit venting is generally a good idea. I've done it this way in the past and I'm a little embarrassed that I never even considered the fact that a substantial portion of the exhaust air could get sucked right back into the soffit vent or adjacent soffit vents.

I do think that by closing off the soffit vents (into the attic) in the joist bay that you're venting from, and one on either side, will greatly mitigate this issue. The moisture should diffuse into the surrounding air very rapidly, especially when there even the slightest breeze. As will making sure the underside of these soffits is painted or otherwise moisture resistant. I also think it's important to realize that this is just bathroom air we're talking about, not a dryer vent, so while the air will be humid during showers it's not like we're blowing clouds of steam.. 

Also keep in mind that the whole point of soffit vents is to allow air to constantly wash along the underside of the roof decking, and remove any moisture from the attic, so any moist air that gets sucked in or condensation happening should be quickly removed- not saying that this can't possibly pose a problem (especially in winter), but I think there's a lot of built-in tolerance.

So I'm somewhat on the fence. I'm leaning towards thinking venting through a gable wall is better, especially if it is protected by an overhang, but I don't think soffit venting is as horrible as some make it out to be. And I would probably choose soffit venting over making another hole in the roof any day, for retrofit especially. I also think Dave makes a lot of good points.