DD is 11 months - we have a Kettler Flipper seat on the back of a bike - a Jamis Coda bought specially to tote her around.
The helmet thing is an issue. I can't seem to find a helmet small enough to fit her. I found a really small one through REI (smallest I could find!) and it still falls over her face. So (eek) we ride without it for now. I feel like a bad parent but I am confident we're going to be fine.
She loves it. Please don't flame me.
2007 Trek 5000
2009 Jamis Coda
1972 Schwinn Suburban
"I rejoice every time I see a woman ride by on a bike. It gives her a feeling of self-reliance and independence the moment she takes her seat; and away she goes, the picture of untrammelled womanhood."
Susan B. Anthony, 1896