This is a very old post, but an interesting topic. I thought I'd post my thoughts to "bump" it up and would love some additional discussion.
For my part, my daughter who is 15 months, asks for "Milk" or "Nurse." I have found signing to be a really great tool for extended breastfeeding, as she'll sign milk to me when she is hungry, which keeps her from just walking up and lifting my shirt at inopportune moments!
I have heard people use "nummies," but in general, I am a fan of calling things what they are.
Nicki