Despite being an apparently nonsensical and vowel-less combination of letters, it seems that wstfgl is a remarkably popular non-word. A search on Google leads to more than fifteen hundred other results, a surprisingly large number for an apparently random non-acronym; for reference, fsahn yields about seventy, fphrg around forty, and yrxkl, fewer than twenty.
There is a reason for this of course; wstfgl, or less often (about four hundred hits on Google), wsfgl, appears by some sort of common consensus to be an accurate representation of the short nonsensical burbling uttered when confronted unexpectedly with a statement or event so ludicrous, unlikely or absurd that it leaves one literally lost of words. According to someone more learned than myself1 its first known occurrence is in Asterix the Legionary, and it is also found in Terry Pratchett's Discworld Novels. Google reveals it to be more than a few people's choice of user-name, and it also has a small vogue on various blogs, fan-fictionsites and and the like.
It is arguably related in some ways to the WTF acronym. For example, for many people, the news that George W Bush got more votes the second time around could have provoked either response.2 Certainly, if you attempt to pronounce WTF, the feeling is similar, but it is not the same; WTF is inherently aggressive and confrontational, wstfgl is merely confused. The two expressions cannot be used completely interchangeably for example, one of the most commonly adopted instances of wstfgl is for the sound made by a person being suddenly shaken or shouted awake. The sudden interjection of reality into what was the perfectly sense-making landscape of a dream does tend to provoke immediate confusion and surprise, but the mental wherewithal required even to think of swearing does not always arrive before an attempt at articulation is made.
It does not even need to be said out loud. The brain is very rarely quiet, comments and happenings that silence it completely are rare, but for those occasions when they do, very shape of wstfgl; the zigzag of the w and s, followed by the loops of the f and g seem to mirror the impression of mental twisting that accompanies the brain's attempt to wrap itself around the perplexing occurrence. This may go some way to explaining its popularity.
1The Discworld Annotator: http://www.lspace.org/books/apf/carpe-jugulum.html
2Just an example, I suspect the news that John Kerry received more votes than many duly elected presidents might have the same effect.