Letters can definitely be considered art - however we usually don't think it of the ones we're most familiar with. In Western societies, people find characters from Oriental scripts pleasing, and turn them into tattoos and such. In China, the reverse is true - they make merchandise (graphic t-shirts, for example) with text in the Latin alphabet, often without any idea what it means, so you end up with children's clothes shops stocking shirts with "fuck" and "i love racism" on them.
Letters, words, phrases ect. are all symbols/placeholders used by societies for emotional and intellectual response, and are thus an extremely valid vehicle to utilise, particularly in contemporary art which places dissemination of concept at a much higher level to aesthetic form (words and phrases are great for their malleablity). Not to say that the presentation of the words doesn't have a profound effect on how the message is conveyed (because it does, and has huge potential to change the whole peice).
In the end, the general response to "is x and x art?" is that anything can be art as long as it is scrutinised, deconstructed and conpared as art. The movement towards conceptualisation and installation has only reinforced this idea and semiotics has become a staple of the intellectual process of desconstruction.
In terms of whether typography can be classed as "good art" though is wholly dependant on how you decide to tackle the concept. calligraphy - adding curly loops onto things itself is fairly shallow, as is fonts ect. it's an artform, but it isn't necessarily art. To forgo the message implied in the actual text to alone look at the aesthetic in which it is presented is ridiculous; you need to scrutinise the entire peice and the aesthetic presentation needs to have a meaningful relationship with the diction of the text involved otherwise it's simply needless pretention of the INT THAT GUD espousers.
If you want to explore the possibilities of using text in art, remember to look at the big picture. Marrying the concepts together (aesthetic, diction and implication) has the potential to make incredibly powerful statements very elegantly.
