This is the United States, where the majority of celebrities can get a second chance, simply by crying on Oprah's couch, but with the economy the way it is, will that be good enough for corporate sponsors, who may be looking for a way to cut expenses?
Learn from this. A situation like this isn't just limited to people with endorsements, it can just as easily affect musicians who have licensed music for corporate advertising.
I'm not making a judgment call about getting completely loaded on booze and smoking marijuana at a college party, by the way. Whatever works for you... But don't bitch when the sponsors you've done deals with, who all have a clause in their contracts prohibiting this kind of behavior, get pissed and pull out.
To end on a good note, the good news for Michael Phelps is that he's just opened up a new segment of sponsors. Zig Zag rolling papers, that guy on the corner selling nickel bags, Cypress Hill...