I just worked through a nagging little assertion error during a NetBeans 6.1 install to Ubuntu 8.04. Hopefully this helps someone.
First I made sure that I had Java 6 downloaded and installed on my 64 bit Ubuntu 8.04 OS.
I noticed the install script scans my hardware for jvms as early as 1.5. Here's the section of netbeans-6.1_ml-linux.sh that contains that information...
Notice I changed JAVA_COMP_VERSION_MIN to 1.6XXX. With the default settings the NetBeans install was picking up 1.5. I was getting Assertion errors and after using the sed option in this thread a blank welcome screen. After changing JAVA_COMP_VERSION_MIN the installer picked up 1.6 and ran fine. W00t! W00t!