Todd is correct that most Burns suppers are considered black tie events, but obviously there is some variation, like Jame's Burns bash in a pub. Our Burns Supper is on the small side, and less formal than most. We are just glad to get men out in their kilts, so we don't make too much of a fuss about the formality of dress. You'll see men in their Prince Charlie's dressed to the nines, and you'll see men in just a kilt and sweater -- and of course, a lot of in-between. The men who don't wear kilts usually wear "Sunday best."
And the variation is even present at the black tie dinners. At the largest Burns Supper I've been to, in St. Louis (average 400-500 in attendance), you will see everything from black tie/PC jackets down to sport coats. At our own Burns Supper, we average around 20 kilted gents with PCs or Argylls, and then the rest are in suits, blazers & grey flannels, etc.

Cheers,

Todd