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First of all the are renaming the service to "Product Advertising API" And that's effective immediate. I've not seen a reason why they are doing this move beyond it reflecting the purpose of the service.
Next we'll be forced to migrate to authenticated services and we're given a timeframe from May 11th till August 15th 2009 to do so.
Unauthenticated requests won't be processed after that date.
Third change is a new ToS:
I'm unsure what exactly changed, but amazon claims they are substantially the same except for the requirement to have authenticated requests (since the unauthenticated ones won't work anymore I fail to see the logic, but ...).
Back to programming after I figure out how to authenticate.
If you didn't program your affiliate stuff yourself, odds are you need to seek out who did and get an update from them to add in authentication or see your application break in August.