Throwing a basket on your bike to carry your office gear is fine. Don't expect to get any positive points from the style police, though. Here's a more elegant proposition: the Linus Bikes Office Bag.
Designed for pedal-pushing office blokes, the bag features straight lines and a structured form that will help you maintain that wholesome, professional image you've worked so hard to establish. Yes, even when you're really a car-hatin' hippie in a suit who eats raw vegetables and drinks amoeba juice for lunch.
The Linus Bikes Office Bag measures 33 x 27 x 11 cm (h x w x d), with enough space to fit a 13-inch laptop and a bunch of accessories inside. Up to two of the bags can attach to a standard bicycle's rear rack during transit (one on either side) using leather straps and brass buckles - the longer the rack, the more you should be able to fasten onto it. An optional D-ring can also be bought if you want to lock the bag onto the rack for more permanent placement (disclaimer: nothing's gonna stop thieves from slicing your bag open, though).
Since you can't take the bike inside the office, you can attach an adjustable strap to the bag, so you can carry it like a regular messenger unit. It's made from durable waxed canvas, either in army green or navy blue, with solid brass hardware parts.
You can score a Linus Bikes Office Bag right from the manufacturer's website, priced at $69.99.