I have two aluminum frame fixed-gear bikes that I ride on the sidewalk 99% of the time--110 PSI in all of the tires. It was pretty rough at first, but nothing I wasn't able to get used to. One of them now even has a hard road saddle.
I had one of the bikes at 90 PSI for a while, but the rolling resistance just killed most of the enjoyment out of it--and I think less than 90 PSI would be unbearable for me.
Maybe you can keep the tires at 90 or 110 PSI, but get a softer saddle?