We can't know for sure. The FDA recently asked for a label change in the erectile dysfunction medications:

A small number of men have lost eyesight in one eye some time after taking Viagra, Cialis, or Levitra. This type of vision loss is called non-arteritic anterior ischemic optic neuropathy (NAION). NAION causes a sudden loss of eyesight because blood flow is blocked to the optic nerve . . . We do not know at this time if Viagra, Cialis, or Levitra causes NAION. NAION also happens in men who do not take these medicines. People who have a higher chance for NAION include those who: have heart disease are over 50 years old, have diabetes, have high blood pressure, have high cholesterol, or smoke . . .

So, what is NAION? Though a rare disorder, it is the leading cause of sudden vision loss in those over 50. There is painless swelling of the optic nerve that causes a rapid reduction in vision. Those stricken with NAION often awake one morning with vision loss, or vision loss may flicker and worsen over one to two weeks. As mentioned above, risk factors include heart disease, hypertension, smoke and high cholesterol.

Medpundit is skeptical of the association - as there have been no reports of NAION in 13,000 people tested in clinical trials for the drug:

NAION is not commonly associated with life-threatening conditions, although the presence of other vascular conditions is frequent (eg, hypertension, 46.9%; diabetes, 23.9%; myocardial infarction, 11%).

I'll be darned. Those are the same diseases associated with impotence. Is it all that surprising some of the people who develop NAION take Viagra? What's surprising is that only 50 of them have been identified so far.

(Hitti, "Label Change for Viagra, Cialis, Levitra", WebMD, Jul.11)