I guess to me it depends on your idea of "drinking."  For me personally I don't worry about it as I don't drink alcohol at all (stuff tastes freakin nasty, all of it!)... but many people worldwide like a glass of wine or such.  In America people tend to think drinking equals the following equation 

any amount of alcohol at all = gonna get drunk

Thats the just the culture we live in, where as when I go to London people have alcohol on a daily basis.  Having a glass of wine before or during a shoot is a very common thing and not frowned upon at all.  Don't get me wrong I'm not saying people are doing funnels, but having a glass of wine before, during the breaks of a shoot or after is completely acceptable in my book.  I mean after all in my studio when clients come in the door I take their coats, direct them to the seating area and I offer them coffee, a bottle of water or glass of wine.  So I certainly don't see anything wrong with that.

One other thing that came to mind.... for those of us that are photographers some of you might be familiar with Joyce Tennesson (hope so).  Not that Joyce and I are best friends, but we are social and occassionally get together for lunch or coffee, etc.  I have also been to Joyce's studio several times and seen her do shoots and both Joyce and the models were drinking wine almost everytime.  Even the assistants were drinking wine, she is a HUGE wine lover.  Now for those of you (models probably) who aren't familiar with Joyce she is a world recognized photographer and named one of the top 10 most influential female photographers ever by Popular Photography Magazine.  So to say any alcohol at all is unprofessional is an unfair statement in my mind.  Like Jeff says "Drinking at all before doing any kind of work is stupid and unprofessional" .... which would be calling people like Joyce Tennesson and many, many of the top European photographers "stupid and unprofessional."  This is obviously not the case, but I think the more appropriate statement would be "drinking alcohol in a non-social manner would be stupid and unprofessional."  Getting drunk is unacceptable, but I see no harm in a glass of wine.

I had a model from California break open a bottle of wine at 11AM for a shoot. it was... Weird. But the images were good and I had my Jedi master with me. Thank goodness nothing controversial came out of that.

When working with certain people - I can see why one would need a few drinks...  I don't condone it, but I can see why... Some of the conversation really lacks substance especially when dealing with someone that talks about themselves during the whole shoot...

Depends on the gig.  When I'm working with models who are also my buddies outside of work, its not a problem and we bust out the vodka early in the evening. 

I swear..I've never touched a bottle.
I see nothing unprefessional about a photographer or model drinking while shooting.   I kind of compare it to being some filthy corporate bastard that leaves the office and throws a few back during lunch.  Damn american mindstate that you can't function properly if you even look at a drink.  what a sham.  I still have no problem telling a model/ make up artist to leave a shoot if they get out of hand. 

Maybe I'm just a lush...
But I'm always drinking something.
I used to have a Bloody Mary every morning for breakfast (usually a double).  My trick for the last year or two is to buy a liter bottle of Coke, dump out half and refill with rum.  Fucking great.  You can be anywhere, and nobody knows... you're just sipping a bottle of Coca-Cola.  God, it's glorious.  So should you drink on a shoot?  Definitely.  Should anyone on the shoot know you're pissed?  Probably not.
Hey... here's one for you. 
Why do you always take at least two mormons fishing with you?
Because if you only take one, he'll drink all your beer.
God.
That's some funny shit...
I guess you've probably gotta be familiar with Utah to think so, though.

I stick with this, I shoot with people I know or respect their art.  I don't drink beforehand if I don't know the photog or feel comfortable.   I NEVER get so drunk I'll do something stupid.  I've shot with alot of people and I've had a nice amount of success.  I won't stop drinking.  I'll continue to cater it to the situation!  But at my 9:00 am shoot I won't be drinking...that would just be weird (Sandy).