This error message is not about the lack of disk space per se (installer checks for that) but about the lack of swap space necessary for fork and exec operation which happens at this point in installation.
You should uninstall this installation and reinstall after making sure that you either cleanup /tmp partition or shutdown all unnecessary processes running on this system or both.
This was a virgin installation of solaris 9 x86. I took all the defaults for recommended disk partitioning.
I skipped the step to install the complete enterprise server package. I downloaded the installer into /var/tmp and I proceeded directly to this install (presumably) with an empty /tmp directory.
So, how large is the swap partition? In my experience, default swap size values set by Solaris end-user configuration installation are usually good enough for end-user desktop system but not for developer's environment where you usually run some sort of server and/or IDE and it is usually good idea to increase default swap space values
Just as a reference value, I remember having similar issues installing Application Server 8 on a Solaris system with 560 MB swap space and 128 MB RAM and installation would go through only if /tmp was perfectly clean and system was running only bare bones processes. Since this is fresh Solaris installation, I would recommend reconfiguring the system with at least 1 GB of swap space - that would give you a nice margin if this system is to be used for J2EE development or production.