Yes, the snowfall effect can be an accessibility issue. Glad to see Ben shared the WCAG 2.0 Level A guidelines.

I commented about the snowfall effect on WordPress.com last year; I was very surprised the feature was released without an option for the user to turn it off, or pause it.

It's a usability issue, in that the user has no control over the effect. I rank it in the same list as audio and video starting automatically when you visit a web page.

Not sure if you've seen it already, but the WebAIM post about seizure disorders provides more information.