Currently hudson ships with the following entries in web.xml
<env-entry>
<env-entry-name>HUDSON_HOME</env-entry-name>
<env-entry-type>java.lang.String</env-entry-type>
<env-entry-value></env-entry-value>
</env-entry>

This entry is only useful if the installation modifies the <env-entry-value>
block and sets a value.

An unfortunate side effect of the default setting is that in tomcat if the user
sets HUDSON_HOME in their hudson "context.xml" that value will only be processed
by tomcat iv the env-entry block is removed.

Many hudson clients prefer to NOT modify the web.xml because it makes it more
difficult to install future hudson versions.

if the env-entry block was inside a comment it would still be usable to users
who modify their web.xml as well as users who don't.