They don't have recomended populations for towns. The only thing that they really have are the town wealth tables in the magic item section.
This same issue drove me crazy a week or so ago.... I spent hours going through the core rule book, back and forth seeking population information that wasn't there... I finally just fell back on the 3.5 DMG table for what I was working on.
Note that this information from 3.5 was not open game content, thus Paizo could not re-release it in the Pathfinder RPG Core Rulebook. What you'll want is the GameMastery Guide, due out in May.
Like the previous poster said, currently they are in the 3.5 DMG. They'll probably cover it in the Game Master's Guide when it comes out.
EDIT: Ninja'd
I had the same issue putting together a large city a little while back. I ended up using the rules from the 3.5 DMG, but I didn't like the results. Particularly because theres such a big gap between the highest level NPCs and the next highest. (e.g. having a 16th lvl fighter as the highest fighter NPC in a town meant that the next highest fighter NPC would be two 8th lvls, but no NPC fighters of 9th to 15th lvl.) Also, in large cities there is a distinct possibility of having ridiculously high level commoners. (1 in 64 metropolises would have a lvl 28 commoner!)
Doing some seraching I came upon the following site that had some articles some guys wrote about the town generating rules and how they would change it. Its a bit geeky of course, but they have some pretty good ideas, so you might want to check it out.
The 3.5 rules that Monte came up with weren't that good to begin with. They were kinda a carry over from the 2e Player Option: High Level Campaigns but they were built around 1e-2e assumptions, ie that levels 8-12 were closer to high level in the game world and that those levels represented a big investment in player time.
The 3.x XP rules made it so that lvl 20 was relatively easy (if not painless) for PCs to achieve but worlds in which only the PCs are high level can present problems : lack of adequate challenges, requires PC crafting and spellcasting, versimilitude, etc.