Its looks like Amazon may have heeded some of the comments around the removal of uploaded MP3’s to your Amazon Library and if you are quick enough can at least save what you have already uploaded once the service is terminated next month.
A quick visit to Your Amazon Account and Music Settings, shows a new option to “keep my songs”. Ticking this should at least retain anything already uploaded. A bit annoying as its means I can’t upload anything new brought via iTunes, but at least the stuff I have already uploaded will still be there.