If employees can e-mail past clients to tell them they've taken a new job, does that mean they can take those clients' e-mail addresses and phone numbers with them when they leave an employer?

No court in California, no published opinion has answered that question.

If information is taken solely for the purpose of sending out an announcement and you can prove that, that may be acceptable to the courts. What the courts have said very clearly is that employees have a right to announce a new affiliation because they have a right to be in business and to move between businesses. Courts don't want to restrict employee mobility. As a consequence, I'm in uncharted waters here, but I think the courts would be prepared to give a certain amount of leeway to employees so long as they were not using customer information for any other purpose.