Neil Jeffers (who, I understand, leaves Oak Hill today) wrote a good short dissertation in his third year on Jeremiah 32:37-41 and infant baptism, and I've just finished reading it. To understand the relevance, imagine the following (imaginary) conversation between a (reformed) paedobaptist and a (reformed) baptist:
Baptist: Oi, stop baptising babies, where do you get that from in the bible eh?
Paedobaptist: Er, no, I'm not gonna stop. Baptising infants of believers is taught in the bible, so it is.
PB: Well, in that whole covenant thing.
B (since he is a reformed baptist): Covenant? Yeah, I've heard of that. Proceed my good man, how does it establish your bizarre watery practices?!
PB: Well, y'know, that thing about the children of believers being included in the covenant along with their parents, and therefore receiving the covenant sign too - circumcision in the old, baptism in the new.
B (being an amicable fellow): Ah, I see. There's one problem with that...
PB (also being an amicable fellow): What's that then my ole chum?
B: Doesn't Jeremiah 31:31-34 teach that the new covenant is different from the old in some crucial ways, ways which would lead to (so to speak) 'knocking baby-baptism' on the old head?
Now, whilst PB would always have had plenty of responses at this point (such as 'hmmm, to what extent do you think Jeremiah 31 is eschatologically fulfilled?'), Neil Jeffers has ably provided another, and that derived from a parallel 'new covenant' passage in Jeremiah:
PB: Good point my good man. However, I fear that Jeremiah 32: 37-41 speaks of the new covenant and includes the children of believers.
B: (mumbles something about Spurgeon and heads for the hills)
Ok, so it's not as straight-forward as that (and obviously Reformed Baptist's would hardly be conquered by a single text). If you want the arguments in all their exegetical and systematic detail then read Neil's dissertation (sorry if you're reading this and not at Oak Hill and therefore can't).
BTW I love my baptist brethren, John Bunyan was a legend, Piper is an inspiration, etc. etc. etc.