how did you get the nekohtml-1.9.8-20080612.144011-4.jar?
I found it in my maven2 repo. I'm guessing that the htmlunit build pulled it in somehow since it appears to automatically download the latest jar.
... [INFO] snapshot net.sourceforge.nekohtml:nekohtml:1.9.8-SNAPSHOT: checking for updates from htmlUnitSnapshots [INFO] snapshot net.sourceforge.nekohtml:nekohtml:1.9.8-SNAPSHOT: checking for updates from nekoHtmlSnapshots ...
Just quick note from myslef. I ran into the same problem. After switching to htmlunit 2.2 with nekhtml 1.9.8 all problems have gone away :).