Really? That sounds fascinating - have you got any references I could follow up on? It's always felt like it should be true, but after acting as if it isn't for a while it didn't seem to hold to me.
The Wikipedia article is quite dry, however, and I suggest this readable NYTimes article instead: http://www.nytimes.com/2008/04/02/opinion/02aamodt.html
Or this one: http://well.blogs.nytimes.com/2007/12/06/how-to-boost-your-w...
That second NYTimes link mentions that the mechanism of willpower depletion might be glucose depletion. Drinking a sugary drink restores willpower. (This effect has also been shown to be true of the self-control of dogs: http://www.miller-mccune.com/health/dogs-offer-clues-to-self...)
Regulating your emotions also depletes willpower (http://www.eurekalert.org/pub_releases/2009-09/mu-rda092409.... but it doesn't go into detail on the regulating emotions part). I have observed that tolerating irritating people harms my productivity.
I warn that at least one study has failed to replicate the willpower depletion result: http://lesswrong.com/lw/1fe/antiakrasia_technique_structured...
Personally I'm skeptical of most of the conclusions I've read about ego depletion. I think it's like anything else: there's a balance to be found between self control and self release. And the more you work at it, the more you can yield and still stay balanced.