[madscientist]

I learned from a previous batch to keep it steady and just turning over.  On said batch, I had a pretty vigorous boil and when I went to add the hops... well let's just say I'm still cleaning hop residue and charred malt extract out of my burners and stove.  Funny thing is, even though I lost about a gallon or so of wort, that beer was my best homebrew to date.  All my friends have asked for it again.  (It was a witbier).

[bonjour]

Actually I'll boil either hard or soft depending on what beer I'm brewing.
On a pils or other beer where I want the lightest color that I can get I'll boil with minimum vigor, on a Barleywine or wee Heavy I go for the hardest boil I can get.

[Mikey]

most brewpubs are exhausting the boil thru an opening about the size of a turkey fryer.  This has to limit evaporation rate.

Unless you are actually building a positive pressure or it's condensing in the flue and running back into the kettle, I don't see why it would.

Exactly,  I've never seen a heated lid and exhaust on a brew kettle to stop condensation.
I knew you knew your physics.

I have. It's simply a matter of channeling the condensation away from the kettle, which most do. This is done not only for the unnecessary water that will be introduced back into the kettle, but for sanitation purposes. Flues aren't cleaned very often and can get pretty nasty. 

[Kaiser]

most brewpubs are exhausting the boil thru an opening about the size of a turkey fryer.  This has to limit evaporation rate.

Unless you are actually building a positive pressure or it's condensing in the flue and running back into the kettle, I don't see why it would.

It actually doesn't matter how wide the venting opening is as long as the steam can get out and you keep adding the heat to the wort that is needed to achieve the targeted boil-off rate. If there is heat loss at the lid b/c of the condensation that heat would have to be added back to wort through increased heating or the condensate needs to be diverted away from the kettle. As others pointed out, the latter is a good idea.

You can also restrict the venting opening at your kettle at home by covering it partially with a lid. If you don't turn down the heat I predict that you'll have a massive boil-over. If you turn down the heat you also lower the boil-off rate.

Kai