W. T. Lee and M. G. Devereux [1] review the bubble formation processes in carbonated drinks,  like fizzy pop or champagne and compare this with heavy stout drinks.  Stout beers have lots of dissolved nitrogen and this makes the physics slightly different to the carbonated drinks.  Specifically, although the same mechanisms apply the time scales are very different.  Stouts will not spontaneously form a head of foam. This means that a widget or similar needs to be added to cans in order to  aid the nucleation of gas bubbles.

In the paper both the mathematical and experimental issues are discussed.  I recommend you use the paper as a conversation starter next time you are wasted on stout at your local ale house.

Bottoms up!

References

[1] W. T. Lee and M. G. Devereux. Foaming in stout beers.  arXiv:1105.2263v1 [physics.chem-ph]