My wife does contract web and social media work for a growing company. That company happens to be the same company I work for and we have equity in. The CEO contracted with her to do behind-the-scenes work for corporate sites. She's now being asked to provide support and consulting for our clients as well. Her role has significantly increased in both duties and hours, going from something she could work on around the kids' schedule to something that is an on-call role, client facing.
She's emailing the CEO (my boss, founder, majority owner) her intention to raise her rates on the next invoice. The CEO is notoriously cheap and negotiates with vendors quite ruthlessly.
Objectively, the new rate she's wanting to bill at is fair. The rate she's billing at now is definitely less than they would get elsewhere.
ETA: the leverage is that she has all the account passwords and is the only person who knows how to implement the local web strategy.