Try also changing your permissions for the upload folder:
/wp-content/uploads/2012/04
I had the same problem this morning…. turns out to be a permissions problem.
Change the following permissions to 777 for the following files:
In the wp-admin folder
/media-upload
/media
/media-new
/load-scripts
And the folder (including it’s files) wp-includes/plupload
Then change them back to 644!!
Me three! Otherwise I love the plug-in.
