Martin Belam (aka @currybet) has written up his guidelines for a corporate Twitter usage policy. Now I've never worked for a company with the nous to have any sort of social networking policy (good, bad or otherwise), but in my current role at Rattle, I do advise companies on how they should best behave in social spaces online.
The guidelines Martin's drafted contains a list of 5 point companies should follow at a minimum. They're pretty sensible, but they're all about what not to do, not what you can or should do. I thought I'd rectify that with another 5 points to add:
6. Do feel free to talk about your work
Whilst employees should feel no obligation to tweet about work (their Twitter account belongs to them, and isn't a channel for their company to do marketing through), they should feel that they can if they want to, so long as they're not leaking any top company secrets. And they shouldn't have to 'ask first', as that just removes any sense of spontaneity, which is what Twitter is all about.
7. Have an official company Twitter account, so that people can reference it
Maintaining a useful, but non-spammy stream of tweets in an 'official' company Twitter stream can be hard work, and might not be right for many companies. But it's still worth having a official account, if only so that people can reference it when talking about the company. This is one of the main reasons I set up our @rattlecentral account - it means that rather than just using 'Rattle' in a tweet, which doesn't mean anything to anyone who hasn't heard of us, or using our URL, which uses up valuable characters, I can just reference @rattlecentral, and anyone who wants to find out more can click through).
8. Subscribe to a search for your company name
One of the really useful features of Twitter is the ability to use search.twitter.com to find out what people are saying about your company. You can even subscribe to the search as an RSS feed. This can tell you a lot about what people think of your company/products, however you should be careful about how you respond to these tweets, if at all. You shouldn't feel obliged to apologise to everyone who complains about your company - sometimes people just like to vent. However if you can offer someone genuine support (say they're complaining about a 'missing feature' which is actually there, but a little hidden), then feel free to help them out in a reply. @firefox_answers is a good example of a totally helpful twitter account.
9. Do feel free to complain about work
Employees shouldn't feel that they can say no wrong about the company they work for. Moaning about work is just part of life - everyone does it. People should feel that they can be honest in the social spaces they inhabit. Of course, serious complaints should go through the official channels (especially if about a another employee), but bitchin' about the quality of the canteen shouldn't be a punishable offense (improve your catering instead).
10. Follow before you tweet
This one is really more of a general Twitter guideline than a corporate-specific one. Basically, if you've only just heard of Twitter (through all the general buzz in the media) but are coming at it fresh, you should spend some time following people you find interesting before you leap in and start tweeting heavily yourself. Every social network has its own conventions, nuances and etiquette, and Twitter is no exception, with a whole set of unique terminology (hashtags, 'RT', @-replies, etc). So start slowly, learn the lingo, and be a consumer as well as a publisher.