We have a business application that basically runs on an os-independent stack (tomcat+java+mysql) but we have always run it redhat or centos.
There is a customer that is insisting to run it on opensolaris for his own reasons (an expensive everything-is-included support agreement with Sun).
How painful can such a migration be? We have a lot of configuration file and support scripts such as:
Our build process outputs a .tar.gz file with our business code + some shell scripts that edit all the os-configuration files.
Any previous experience on this.
The biggest issues will be with the non-POSIX (non-standard) options you've used to the GNU tools provided on Linux that are not in the Solaris standard commands. You might decide that porting the relevant tools from the GNU set is simpler than modifying your system. If you've laced the code with absolute pathnames for commands (/usr/bin/ls) but you decide to use the GNU versions instead, you've got to find a way of fixing those. I'd be extremely cautious about replacing the OpenSolaris versions with the GNU versions; you don't know when you would break something that the system relies on. So, you would put the GNU commands in a separate directory - probably not /usr/local because that is for the machine owners to populate, not you as an application-monger - and arrange for that to be used in place of the system commands. (Note: on Solaris, /bin is a symlink to /usr/bin; I assume the same is true of OpenSolaris.) AFAIK, Postfix is not standard on OpenSolaris, so you'd have to ensure you get that installed, too.
All of this is doable - there's nothing insuperable. But a lot depends on your code base.