The build.xml file which ships with both TomCat 5.5.16 and 5.5.17 (source package) is missing the following: <?xml version="1.0"?> from the first line of the file. I installed the package fine on Fedora Core 4 (Stock install), but apparently the XML handling on Fedora Core 5 is much stricter and leads to a NullPointerException as a result. This file needs to be updated to include this line. I used the same Java SDK on both.
The xml specification recommends this but does not mandate it. build.xml is compliant with the xml spec without this text. The root cuase of this issue is the overly strict xml validation on your system. I am therefore changing this to an enhancement request and have changed the title to since build.xml is not invalid.
I agree with Mark's opinions, and I think this is a worthy enhancement, so I've gone ahead and added the XML declaration to the main build.xml file and a couple of other intermediate build.xml files as well. Thanks for reporting this.