[caf]

I dunno, that's definitely on the verge of working as a name. You can certainly imagine that as a nickname.

[blowski]

Yep - Douglas Squirrel (http://douglassquirrel.com/) is a guy in London who does a lot of work with startups, and he is mostly known just as 'Squirrel'. Really cool guy, by the way, if you get the chance to talk to him at a meetup.

[317070]

Well, my SO calls me that as a nickname. I have ginger hair and I am kind of impulsive. That and a squirrel came eating out of my hand one of the first times we met.

[mikeash]

A search on dobsearch.com turns up quite a few people named Squirrel. It's far from common, but it exists.