I can't really offer any useful advice on this one, but lots of sympathy - cos my 22month old DD is doing the same thing.
My bub is due in August, and for the last couple of months DD has been obsessed with being treated like a baby
eg: dinner time: "Mummy feed Jess like a baby" ie - she won't eat unless I use the spoon/fork.
moving about: "Jess no can walk", and insists on being carried like a baby.
anytime: "Mummy, babys can cry ""wah wah wah wah ( repeat ad infinitum)""
And as with Nicknoks DD, she keeps wanting to be picking up and rocked and sang lullabies ( not that she would put up with that when she was a baby - all she wanted to hear was the blues brothers !!! :-) )
We're also trying the ignore it approach. So far, no luck but fingers crossed. The other thing we're doing is gvettingher into a few more activities with kids her own age or a little bit older ( eg we've just joined th 2-4 y playgroup). We're hoping that the combination of distraction and seeing what fun the "big"kids are having will do teh trick.