From a management view, it's not quite that simple. First, you'll want to check with your legal team. Then, you'll want to revisit your code of conduct/ethics and related policies and procedures for technology. It is very likely that use of consumer tools like Twitter will be treated as organizations handled public instant messaging tools. If you block consumer IM tools, then you probably probably block Twitter (as best you can). If you limit use of public IM to certain individuals, or roles, then that will probably apply to Twitter accounts - there will be a desire to someone centrally manage the external corporate identity by having a process where workers can register their Twitter names if those accounts are used to represent the business. If you have logging, archival, audit and compliance policies for IM - then they probably apply to Twitter which might be another reason to block it. If you are worried about the names people use for their Twitter handles, you will want to monitor its use, notify workers to change those handles if offensive in some way (if used for business purposes). If they access Twitter from home - and use it to represent the company - a policy-and-procedure should cover that scenario (including warning up to termination).
This is nothing new. Organizations have been following similar steps as described above for many years. If you use these tools to represent your employer, then expect the employer to set guidelines. If you don't follow the guidelines, expect to be warned and perhaps terminated (depending on local and other applicable laws). Organizations have policies and procedures to handle a wide variety situations and behaviors when workers represent themselves in work-related ways that also ends up representing their employer.
This creates business opportunities for companies to offer policy management, monitoring controls and other tools as has happened with other communication and collaboration tools. If you use Twitter without any association with your employer - then perhaps the only issue then is use during work and any perceived risk (e.g., security) that people might believe Twitter poses.
All that said, there are tremendous benefits in terms of keeping up to date on things, building relationships, gaining a sense of community, and so on. That's why there is a market for enterprise versions of social messaging tools. The challenge is the space between the enterprise and the public Internet. Use of consumer tools for some people in some organizations might be perfectly fine - but there are organizations where such use can raise significant concerns. It's best to undertake the proper due diligence and manage expectations early-on rather than let viral use create an over-reaction on the part of leadership teams.
Twitter in the Workplace
Should you allow your employees to use Twitter?
Are they already using Twitter?
The answer to both questions is probably YES.
Your employees are probably already using Twitter and they do not need your computer network for access. They can access Twitter by text messages, an iPhone, a blackberry or any other smartphone.
Personally, I have been using Twitter for a while and find it a great way to stay connected with the news, compliance issues and the other compliance related people using Twitter. ( See @DougCornelius on Twitter)