I am also out at work, and I agree that it's extremely important... but I do understand that it's sadly just not possible for some people and some working environments. My company is extremely liberal, donated money to fight Prop 8 and includes both sexual orientation and gender expression in its nondiscrimination policy.
Even though I've never made it a secret, occasionally I am surprised to learn that a coworker has worked alongside me for two years and had no idea. Some of my friends joke that I'm a "stealth lesbian," even though I wear little rainbow ring earrings every day... so I recently had thin rainbow bands tattooed around my left wrist, also, of which I am very fond. I'd like to say it helps me be more visible, but I guess that remains to be seen (accidental play on words, but I'm keeping it)! A continuing source of frustration for me is my seeming invisibility sometimes... it's difficult to let all these wonderful butches know I'm here.