Chris, hairline cracks in your new basement floor are almost a given. Now, the builder saying they were "done on purpose" sounds a little flakey to me, unless they cut what's called "control joints", which is basically what you see in sidewalks and driveways, and you would have seen these from the very beginning. Expansion joints will be complete separations with a fibrous, flexible filler strip, and you would not normally find one of these in a basement. A control joint is merely making a groove in the surface (often with a radiused edge) in an attempt to "force" the crack which might occur in the future into a specific place (in the control joint) so that the crack does not readily show and any minor displacement will not cause problems. Sometimes these are done in basement floors, but many are poured with no control joints at all.
Also not uncommon would be finding a few new cracks as time passes, but again I'm speaking of hairline cracks which do not show major signs of offset displacement or separation.
Most likely, what you have would be in the normal range, however, Tom is correct in saying that without seeing the problem I couldn't be 100% positive of whether this is within normal limits or not, so if your very concerned I would have it looked at by an independant party.