Re: Swap filling up but not emptying.
What programs are you running? It definitely sounds like a memory leak, unless you are doing some major graphics or video editing or ripping disks while using tmpfs. The only memory leak I have seen in years was when I was running 11.04 beta and its network manager was leaking.
I run Ubuntu (although 64-bit 12.04) from a 2 core 1 GHz tablet PC with 2 GB RAM booted from SD card using tmpfs (in RAM) for /tmp without any swap. It cannot hibernate because its SD slot is internally USB connected, so it does not need swap for that.
My desktop PC (up 20+ hrs) is currently using 765 MB with firefox and a terminal open. The 4.9 GB cached data is from running tf2 game in Steam earlier.
efflandt@xps8100-1204:~$ free -m
total used free shared buffers cached
Mem: 7913 5783 2130 0 85 4931
-/+ buffers/cache: 765 7147
Swap: 0 0 0
i5 650 3.2 GHz, 8 GB, nvidia GTX 550 Ti @ 1080p (year old GT 430 began freezing) and assorted other computers