Caffeine, good for man, great for yeast?

Caffeine + yeast, does it inhibit or promote yeasts effectiveness at fermentation? According to this, it extends their lifespan, but does that extension actually benefit our brews? What other effects might caffeine have?

You can make Buzz beer A Whole New Buzz - Caffeinated Beer

Has anyone done this with homebrew? Anyone add Guarana? I think it would be kind of cool to have a nice light lawnmower beer with a caffeine kick.

Guarana is just a plant. It happens to contain caffeine (just like coffee beans). It contains about twice the amount than a coffee bean though. Besides coffee stouts/porters, I don't see any reason to add caffeine to beer. The effects of caffeine in said stouts/porters are probably negligible anyways.

I'm more curious as to the effect on yeast, whether its detrimental or beneficial to their being and production/reproduction.

Anyone have access to this journal? I won't be on campus for a while.

i know there has been lots of hoop da do in the mushoom growing world about the benafits of adding coffee grounds to grain spawn and adding coffee to the irrgation water with really positve results.
yeast being a fungus i dont see how it could hurt
few things mold faster than coffee.

This might be a topic more suited for the "Brewing Science" section of the board but my opinion on this is that it is unlikely that caffeine would have much effect for the brewing process.

The life extension referred to in that journal abstract refers to the number of generations that yeast will mitotically divide before they reach senescence (i.e. they become too old to divide).

This really has no practical implications for brewing/homebrewer. Yeast do mitotically divide as part of their life cycle when pitched into a batch of beer, but its a limited amount of divisions. I don't know how many exactly divisions, I would guess between 10-20 divisions before they reach the appropriate density and switch over to anaerobic growth (i.e. Fermentation/alcohol production).

Caffeine may be of use to a company like White Lab/Wyeast, where they deal with yeast strains undergoing many, many, many cycles of mitotic division as part of their yeast propagation scheme.

This is just my spitballing knowing what I know about aging/life extension research and yeast model systems...