I'd be interested in hearing more responses - I'm working my way through the Coming Out thread, which has lots of good stuff in it.
My kids are still a bit younger, but it's pretty clear that AM is a big part of our life, right now. We're mostly good on hiding our affection for her, but the way we've been able to keep the kids from thinking that Dad has something with Someone Else is that Mom talks about the same someone else with the same affection in her voice. The kids take this all in stride.
Still, we recognize that we'd like to tell the kids before they walk in on us, or wake up during one of our (sometimes noisy, oops) bed moments and wonder "why are there THREE voices?!?" Right now, as we're mostly in the infatuated and dating stage, we're not telling them. But given the right time and circumstance, we know it's going to be the right time to do it...