Tested on netbeans 6.8, netbeans 6.9 Beta, Dev build 201004260200
Using Ubuntu Linux 2.6.31-20-generic
Xdebug 2.1.0rc1 (also tested on 2.0.4 and 2.0.5 doesn't work)
All have same issue.

How to reproduce:
Start debugging session set cursor to somewhere in the file and press "Run to cursor" Nothing happens  if any of step.. buttons pressed 1-2 times exception is produced in IDE log:

INFO [org.netbeans.modules.php.dbgp.DebugSession]
java.net.SocketException: Broken pipe
        at java.net.SocketOutputStream.socketWrite0(Native Method)
        at java.net.SocketOutputStream.socketWrite(Unknown Source)
        at java.net.SocketOutputStream.write(Unknown Source)
        at org.netbeans.modules.php.dbgp.packets.DbgpCommand.send(DbgpCommand.java:84)
        at org.netbeans.modules.php.dbgp.DebugSession.sendCommands(DebugSession.java:205)
        at org.netbeans.modules.php.dbgp.DebugSession.run(DebugSession.java:139)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
        at java.util.concurrent.FutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
[catch] at java.lang.Thread.run(Unknown Source)
INFO [org.netbeans.modules.php.dbgp.DebugSession]
java.net.SocketException: Broken pipe
        at java.net.SocketOutputStream.socketWrite0(Native Method)
        at java.net.SocketOutputStream.socketWrite(Unknown Source)
        at java.net.SocketOutputStream.write(Unknown Source)
        at org.netbeans.modules.php.dbgp.packets.DbgpCommand.send(DbgpCommand.java:84)
        at org.netbeans.modules.php.dbgp.DebugSession.sendCommands(DebugSession.java:205)
        at org.netbeans.modules.php.dbgp.DebugSession.run(DebugSession.java:139)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
        at java.util.concurrent.FutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
[catch] at java.lang.Thread.run(Unknown Source)
INFO [org.netbeans.modules.php.dbgp.DebugSession]
java.net.SocketException: Broken pipe
        at java.net.SocketOutputStream.socketWrite0(Native Method)
        at java.net.SocketOutputStream.socketWrite(Unknown Source)
        at java.net.SocketOutputStream.write(Unknown Source)
        at org.netbeans.modules.php.dbgp.packets.DbgpCommand.send(DbgpCommand.java:84)
        at org.netbeans.modules.php.dbgp.DebugSession.sendCommands(DebugSession.java:205)
        at org.netbeans.modules.php.dbgp.DebugSession.run(DebugSession.java:139)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
        at java.util.concurrent.FutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
[catch] at java.lang.Thread.run(Unknown Source)


P.S. netbeans doesn't stop on breakpoints too.