Skim coating textured walls
I've done this on one room- taking a knock-down texture and skim coating it to make it smooth. 

The downside is that you then are able to see every wave, bump, and bobble that the drywall installers didn't bother fixing because they knew it would be hidden by texture. ;)



 

I was able to get by with one coat and one sanding, but I was also applying a textured paint, so I might have had to do a second coat, otherwise.