Serial port reported in use when uploading - on OS X Mountail Lion GM

I have installed the latest OS X Mountain Lion GM. Since then, when I try to upload a sketch from within the Arduino IDE I get the following error message.

Binary sketch size: 15,272 bytes (of a 30,720 byte maximum)
processing.app.SerialException: Serial port '/dev/tty.usbserial-A6005lxn' already in use. Try quiting any programs that may be using it.
at processing.app.Serial.(Serial.java:171)
at processing.app.Serial.(Serial.java:77)
at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:77)
at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:172)
at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:67)
at processing.app.Sketch.upload(Sketch.java:1706)
at processing.app.Sketch.exportApplet(Sketch.java:1662)
at processing.app.Sketch.exportApplet(Sketch.java:1634)
at processing.app.Editor$DefaultExportHandler.run(Editor.java:2346)
at java.lang.Thread.run(Thread.java:680)
processing.app.debug.RunnerException: Serial port '/dev/tty.usbserial-A6005lxn' already in use. Try quiting any programs that may be using it.
at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:101)
at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:172)
at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:67)
at processing.app.Sketch.upload(Sketch.java:1706)
at processing.app.Sketch.exportApplet(Sketch.java:1662)
at processing.app.Sketch.exportApplet(Sketch.java:1634)
at processing.app.Editor$DefaultExportHandler.run(Editor.java:2346)
at java.lang.Thread.run(Thread.java:680)

I can successfully open the same serial port with other applications. I have made sure that the serial port is not opened by any other application when I try to upload my sketch.

I'm guessing this has something to do with OS X Mountain Lion as I didn't have any problems with Snow Leopard.

Run these commands:
$ sudo mkdir /var/lock
$ sudo chmod 777 /var/lock

Thanks James,

I managed to fix the problem yesterday. I found older versions of the RXTX library files in the /Library/Java/Extensions directory. They obviously were used instead of the ones in the Arduino installation directory. By removing the old files it suddenly worked again. It's a bit weird that I haven't had any problems before upgrading to ML though.

I'm a very amateur Arduino user. In fact I 've just purchased my first ever board (a mega 2560)
I'm working on a project for uni and Arduino is the perfect solution, however, I have limited information about how it works and am basically educating myself from home via the forums and tutorials.

I was able to understand the schematics and tutorials but had this problem as well. The board works on 2 other laptops with Mountain Lion installed.
I have followed threads I have found about this topic but have been unsuccessful in correcting the problem. When I plug the mega in I never get an option to select my serial port, it just doesn't appear, and when I compile the code it gives me an identical warning to the one above.

I'm running on a macbook pro i7, 8gb ram, mountain lion 10.8.2.

I have restarted the computer several times, have tried a number of different versions of the IDE, have also downloaded and installed the latest drivers from FTDI chip. (Although I know certain versions of arduino don't require drivers, it appears from the threads I've read, which you have commented on James C4S) that mountain lion requires drivers for arduino now?

I've also had a go at dropping that line into the terminal app without getting a response ( I've attached screen grab of this)

The serial port option of "tty.usbmodem" doesn't work either...

I have a partition and run windows as well and I have the same problem on that side.

I'm can't locate any java folder in my library folder either.

Screen Shot 2012-10-10 at 4.57.39 PM.png

It would be better to copy and paste the command because I used the "pipe" character and not the letter l.

lsof | grep usbmodem