If you download 64-bit Phun for Ubuntu 10.04, it probably won't work:

:) ./phun
There are missing dependencies.
Please make sure that all the required libraries are installed.
Missing:
  libSDL_image-1.2.so.0 => not found
  libpng.so.3 => not found

But even after you install libSDL and libpng3, it still won't work:

:) aptitude install libsdl-image1.2
:) aptitude install libpng3
:) locate libpng
/lib/libpng12.so.0
/lib/libpng12.so.0.42.0
...
[no-where is libpng.so.3!]

To fix this, just symlink /lib/libpng12.so.0 up like this:

:) sudo bash
# cd /lib ; ln -s libpng12.so.0 libpng.so.3
# ls -al *libpng*
lrwxrwxrwx 1 root root     18 2010-09-27 08:47 libpng12.so.0 -> libpng12.so.0.42.0
-rw-r--r-- 1 root root 158736 2010-07-05 10:55 libpng12.so.0.42.0
lrwxrwxrwx 1 root root     13 2010-09-30 11:12 libpng.so.3 -> libpng12.so.0

See that third entry, libpng.so.3? That is what Phun is looking for, but libpng12.so.0 seems to work just fine. Now I can start Phun and use it!