I've heard that adding cleaned pennies to the wort can help prevent boil-overs. I guess they could provide nucleation points- I dunno.
Any truth to this, or is it just an old wives' tail?
In the fourth & final Brew Strong episode on high gravity brewing, Jamil, John, and Tasty touched briefly on using pre-1981 pennies in the boil to help provide minerals (Copper, Zinc) to the yeast to help them survive the stress of a big fermentation. (They mention it around the 30:00 mark.)
The effect that you are describing is that of adding a nucleation point (or site) to the kettle, as you said. A nucleation point provides a place for bubbles to form and release and will help to prevent boilovers. That being said, though, a penny (or a few pennies) will not have enough surface area to help a batch that large. At places that sell cooking gadgets, you can get a ceramic "boilover preventer". I have used these for cooking, and they work pretty well.
As for brewing, though, I have to recommend Fermcap-S. It is one of those things that once you use it, you will never know how you brewed without it. The cost is very minimal, when you determine how long a bottle lasts you. Last night's brew session, I realized I didn't have any and started without. I was having really bad foaming issues that was causing me to back off the boil, so I added a couple of Gas-X pills. In a pinch, this is something I have done a couple of times, and it works well - although not as good as Fermcap-S. Cost per batch will easily fall into favor of Fermcap-S, as well, when comparing the two.
to prevent boil overs, I learned from my grandmother, use a boiling disk. It is a stainless steel, glass or ceramic disk with two notches in the edge. you put it in the pot at the start when the water begins to boil it "clatters" warning you the boil has started. It also breaks up the bubbles that form at the bottom of the pot and disperses them so you don't get those BIG bubbles exploding at the top, splashing liquid all over. Also called a "pot minder" sold on amazon ( about $5), Lehmans (glass about $2)
it works and can be sterilized and doesn't add chemicals to your brew.