With mvm b08 (jdk7-b92), the test fails as follows:

#Test Results (version 2)
#Wed Jun 02 14:10:45 BST 2010
#checksum:26380f889bccfdc1
#-----testdescription-----
$file=C:\\ws\\mvm-jdk7\\jdk\\test\\java\\io\\pathNames\\win32\\DriveOnly.java
$root=C:\\ws\\mvm-jdk7\\jdk\\test
keywords=bug4096648
run=ASSUMED_ACTION main DriveOnly\r\n
source=DriveOnly.java
title=Make sure that isDirectory and lastModified work on "x:"

#-----environment-----

#-----testresult-----
description=file:///C:/ws/mvm-jdk7/jdk/test/java/io/pathNames/win32/DriveOnly.java
end=Wed Jun 02 14:10:45 BST 2010
environment=regtest
execStatus=Failed. Execution failed: `main' threw exception: java.lang.Exception: "C:" is not a directory
javatestOS=Windows XP 5.1 (x86)
javatestVersion=3.2.2_02
script=com.sun.javatest.regtest.RegressionScript
sections=script_messages build compile main
start=Wed Jun 02 14:10:44 BST 2010
test=java/io/pathNames/win32/DriveOnly.java
work=C:\\jtreg\\JTwork\\java\\io\\pathNames\\win32

#section:script_messages
----------messages:(1/68)*----------
JDK under test: c:\\ws\\mvm-jdk7\\build\\windows-i586\\j2sdk-image

#section:build
----------messages:(3/97)----------
command: build DriveOnly
reason: Named class compiled on demand
elapsed time (seconds): 0.593
result: Passed. Build successful

#section:compile
----------messages:(3/172)*----------
command: compile C:\\ws\\mvm-jdk7\\jdk\\test\\java\\io\\pathNames\\win32\\DriveOnly.java
reason: .class file out of date or does not exist
elapsed time (seconds): 0.593
----------System.out:(0/0)----------
----------System.err:(0/0)----------
result: Passed. Compilation successful

#section:main
----------messages:(3/120)----------
command: main DriveOnly
reason: Assumed action based on file name: run main DriveOnly
elapsed time (seconds): 0.141
----------System.out:(0/0)----------
----------System.err:(13/711)----------
java.lang.Exception: "C:" is not a directory
at DriveOnly.main(DriveOnly.java:45)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:613)
at com.sun.javatest.regtest.MainWrapper$MainThread.run(MainWrapper.java:78)
at java.lang.Thread.run(Thread.java:717)

JavaTest Message: Test threw exception: java.lang.Exception: "C:" is not a directory
JavaTest Message: shutting down test

STATUS:Failed.`main' threw exception: java.lang.Exception: "C:" is not a directory
result: Failed. Execution failed: `main' threw exception: java.lang.Exception: "C:" is not a directory


test result: Failed. Execution failed: `main' threw exception: java.lang.Exception: "C:" is not a directory