Unless you are a strict vegetarian (vegan), you can obtain B12 from eggs or dairy products. Most vegans are well aware of the danger of B12 deficiency, and either take supplements or eat foods fortified with B12. B12 supplements are inexpensive and widely available. So how exactly is B12 a serious problem for vegetarians?
There's also Marmite, which, according to wikipedia, is suitable for vegans (I'm a meat-eater though, so I have no idea really). I grew up eating practically nothing but Marmite sandwiches - it's awesome stuff: http://en.wikipedia.org/wiki/Marmite
