Some multiplatform installers I know of are :
IzPack is a an open source software installer generator. As it uses Java you need the JRE on the target platform.
The free InstallJammer is :
a multiplatform GUI installer and builder designed to be
completely cross-platform and function on Windows and most all
versions of UNIX with eventual support for Mac OS X.
You could contact the developer, as he is very responsive.
InstallAnywhere is a commercial multiplatform installer. Its Standard Edition is priced at $1,999.
BitRock's InstallBuilder (don't look at the price tag) also claims :
Generate installers for all target platforms from a single project
file. Create all your installers from a single build environment!
JExpress is another one at only $849.
The only one from the above that I have used is InstallJammer, which I really recommend. If OS X is already available in beta, don't hesitate to try it, as the developer corrects bugs at a very fast pace.