I drank A LOT before I found out I was pregnant, but I was only about three weeks along when I found out so, although there is potential for harm, at least I was able to limit it from that point on (I haven't drunk at all since, but I'm one of those who can't see the point of drinking a little bit - I only drink to get pissed).
I personally think that common sense (and a bit of research if necessary) should prevail in matters such as this. Guinness, red wine and stout do NOT contain anywhere near enough iron or anything else to make them beneficial during pregnancy, for a start, and anyone can (and should, if they're pregnant, really) look this up in a quick internet search before they decide to fill their boots. Also, when we drink alcohol, it is absorbed through the skin in your throat and stomach lining directly into the bloodstream (hence why you can get pissed by pouring vodka into your eye or soaking a tampon in it, if you're feeling so inclined), and obviously from the bloodstream it is passed through the placenta and to your baby. In the early stages of pregnancy, or if you drink a fair amount or very quickly, this can obviously lead to ill effects in your developing baby. Now, if you really feel the need to drink, then in the later stages of pregnancy, one or two units occasionally (which, as someone else mentioned, is just one very small glass of wine) should do no harm - after all, if you had your baby at thirty-odd weeks and (for some reason) gave them the same amount to drink themselves, there would be no lasting ill effects on the baby.
Basically, unless you're going out and getting shitfaced every friday night, I think it's up to you what you do, and I would never think anything of a pregnant woman enjoying a glass of wine on her birthday or half a lager on a summer's night. But I do think carrying a child means you should cut down if you're used to drinking heavily or regularly. I know it's been said already, but it is only nine months, it's not the end of the world.