Diabetics are concerned with the glycemic index of the foods and drinks they put into their bodies, because the glycemic index has a direct effect on the insulin-response (ie how much insulin is released in response to the food ingested).
A lot of information you might read will suggest that beer has a low glycemic index because it actually has very little carbohydrate in it. However, some beers have sugars in them than others (try some of the real ales in the UK, or the Trappist beers from Belgium, and you’ll taste a variety of beer with differing levels of sweetness). The level of sugar in the final beer will depend on how much of the sugars are converted to alcohol during the fermentation process.
Beer is brewed by turning sugar into alcohol during the fermentation process. This fermentation process is carried out by yeasts, which carry out anaerobic respiration using the sugar as energy. The fermentation process can be stopped at any point by lowering the temperature of the beer as it ferments, killing off the yeast cells. Some beers will therefore have more sugars in them than others and will be sweeter, while those that are allowed to fully ferment will be dryer. Since sugar still exists in the beer, its glycemic index is rarely zero as some report. On top of the sugars present in the beer, the alcohol also has a lot of calories, which is not good for your waste line.
What this all means to diabetics is that beer will actually stimulate insulin release. Dryer beers will have less of an effect on insulin levels than the sweeter ones (which should be avoided).
In general, beer is something that diabetes sufferers should avoid in their diet. Be aware as well that non-alcoholic beers are not good either, as they often have higher levels of sugar (or malt) added to make them taste better.