I built a plugin today that disables the snow by default and allows users to opt-in by clicking a snowflake shown in the header of the site.

It's up on github: Jetpack Holiday Snow Opt-In. Give it a test and let me know what you think.

I'll submit to the repo once I've been able to test it a bit more, as this is pretty alpha. I'm also developing a Chrome Extension that users can install to block the snow JavaScript file from being loaded on any site.