im wondering the same thing atm. DD is 15 months, but i think we will wait a few more months till TTC.
My reason is that i want to enjoy DD on her own for a while longer and then be able to enjoy #2 when 'it' comes, and i feel that if i had them too close together i wouldnt be able to do this.
Also i think it depends on the way you parent to a degree, i feed DD to sleep, couldnt imagine having two that needed to be fed to sleep!! how would that work??
I think you adapt to what ever happens though, im sure if i got pregnant now it would be just fine!!
Its hard to ignore that little voice in your head that wispers 'go on....try now!" lol

its interesting reading everyones comments re: having them closer! making me think twice lol....dont tell DH