Is a company singular or plural?

My recent post about the Economist Style Guide included a passing comment about whether companies were singular or plural.  It generated a surprising number of comments, and some grammarphile controversy.

It turns out singular vs plural is a very important aspect when learning how to write and edit.

Companies: singular or plural?

The Economist says they're singular. I agreed.

But today (coming back from a week's holiday in Goa), I've come across a situation which leaves me scratching my head - a two-person company.  Writing about them as a singular entity seems oddly formal and doesn't look right at all.

It just goes to prove the old adage: 'if in doubt, prefer geniality to good grammar'.