Verify that you are indeed using the JDK. You can do this by becoming root or using sudo to check the "alternatives" program. By default Fedora uses gcj. You can tell it to use the JDK by properly setting up your alternatives.
[root@localhost ~]# alternatives --config java
There are 3 programs which provide 'java'.
Selection Command
-----------------------------------------------
* 1 /usr/lib/jvm/jre-1.4.2-gcj/bin/java
2 /usr/java/jdk1.6.0/bin/java
Enter to keep the current selection[+], or type selection number: 2
[root@localhost ~]#
You may have to tell alternatives that another java version exists. The instructions are well defined in the alternatives man page. You'll need to do something similar for jar and javac. Make sure the path is proper for your JDK installation.
Updated 8/27/09
It seems that this is a well known issue with Java & Fedora 8.
Can't install Netbeans on Fedora 8
Here's the workaround from the netbeans mailing list...
Netbeans.org workaround