Hearing rumors of thinner and lighter macbooks and the 120gb ipod classic would lead me to believe the size reduction is going to be a result of moving from a 2.5" to a 1.8" HDD for the laptops since 120gb is pretty good for notebook capacity.
I don't think so either, the smaller 1.8" drives are way slower also, only 4200rpm, while compared to 7200 or 5400 rpm for the 2.5" drives. This means boot times are slower and things like converting video will slow down.
A computer can only be as fast as its slowest part...
And to answer the original question, if the new models replace the current Macbook, which i believe they will, the price shouldn't change much.
There are a few events approaching that they may be released at. Might be with Leopard in October or else at Macworld in January...
I hope October though...