Sometimes answers to your question can happen many minutes, hours, or even days after you asked the question. In the case of days, the e-mail notifications are certainly enough (and just noticing the site), but many of my questions end up not having answers for a good 20-30 minutes.
I'd like to be notified of new answers by SMS (text message), the moment the answer is posted. I always have my cellphone with me, and I already use it for PUSH type notifications from other things (including emails, with auto-forwarding to my cell number @vtext.com). I would just use the auto-forwarding trick with SO too, except that it doesn't offer the option to send an email with each answer, the moment the answer is posted.