I am aware there are a couple of threads talking about the upcoming update of the A500 to 3.1 from various sources, but this was just posted by Acer itself on its Facebook page:
Acer
Great news for the lucky owners of Acer Iconia Tab A500! The update to Android 3.1 will be available at the end of June! Stay tuned for the date of release, in the meantime see the new features in the official Android developers

Great news for the lucky owners of Acer Iconia Tab A500! The update to Android 3.1 will be available at the end of June! Stay tuned for the date of release, in the meantime see the new features in the official Android developers