The order of columns in a table
The on-disk structure of a table is independent of the order of columns the CREATE TABLE. This means that performance is not affected.

Each entry (this applies to both data and index) looks like


Header
Fixed length columns
Null bitmap
Variable length columns
Row version


For more, see Inside the Storage Engine: Anatomy of a record

There are a few cases where it can matter. But you won't usually find them day to day: normaly in repro scripts you'll find here or in blogs. For example, see comments to my SO answer here: https://stackoverflow.com/a/6121952/27535 and the link in comments below by @SQL Kiwi

However, from a consistency, maintainability and readability, I'd have PK columns first with columns in the same order as the actual PK key columns. Saying that, I don't normally rebuild tables to ensure that columns are in the "correct" place. It really does make little difference except to my OCD.