… a pint of water.
I had thought of the question before, and was reminded by this thread on the diuretic effect of tea. Answers there say the effect varies quite a bit, depending partly on the tea’s content of caffeine and related chemicals, and partly on how well hydrated you are to start.
I have the same question about beer. How much water do really gain or lose when we have a pint? Of course the answer will vary, as it does for tea, but is there a general range to be expected?
According to this, you lose about 10 ml of urine for every 1 ml (or 1 g according to another place) of pure alcohol.
Ah, crap, I meant mL of course. This might be of interest to you too. It’s a pdf, but it’s from the WHO and around page 11 is where it talks about research done on dehydrating effects of water. Warning: it is a PDF (116kb) Linkity
So according to that formula, drinking beer of less than 10% alcohol by volume won’t dehydrate you. So stay away from the barleywines and Sammiclaus and you’ll be fine! 
Note that this implies that if you drink a beer that’s less than about 9% alcohol by volume (which most are), then you won’t end up dehydrated. You won’t retain as much water as you would if you just drank water, but you’re not going to end up “behind” on the transaction either.
Man, stay away from the Sammiclaus regardless. I had that as my last drink on a bender I shared with a girl from class, and all I remember from that night was “hailing taxis” (read: running into the middle of the road and stopping strangers at 3 AM) to take us back to her place. That was a hangover.
Thanks for the links, audiobottle. That second one is just the sort of thing I was looking for.
Really, though, I think the best piece of advice offered here is to stay away from the Sammiclaus. I’ve got a headache just thinking about it.