An old joke about San Francisco's economy is that half the people are in the business of selling t-shirts to the other half. Any Valley denizen quickly accumulates a wide assortment of corporate logos in their laundry. But be careful which company's brand you're sporting around the office.
I work for a fairly large ad network that competes with Google Adsense. A couple of days ago, a new employee was sportin' a Google shirt and I was a little upset. What's the protocol on this? People have brought in embroidered bags from the likes of eBay and Yahoo, which is understandable because the logos are smaller and bags have more utility than a t-shirt. We also have our own company shirts available. So what are the rules? Can you represent your previous companies and what if your previous company is a competitor?
The first rule is, wear something nicer than a t-shirt. A pressed, button-front shirt or blouse, for instance. Haven't had time to do laundry? Light sweaters over a wrinkled shirt have saved many a morning. In fact, keeping a light sweater at the office (along with a full change of clothes tucked in a drawer) can save many, many embarrassments, from inappropriate logos to coffee stains are a romp in the janitor's closet.
If you have to wear a t-shirt, be a team player. If you're going to wear a shirt from a previous employer, make sure it's not a direct competitor or a company with better pay and benefits — with turnover what it is in the Valley means managers are constantly on the lookout for disloyalty. Though if you actually have a job offer from the competition, feel free to play it up for a raise.
Other acceptable options would be companies that have tanked, startups you know are hot but your boss hasn't heard of (as long as they aren't challenging your business), something from Threadless, an independent local artist or designer or a concert souvenir from either a new and hot or ironically old band. That is, if it were acceptable to wear a t-shirt to work.