"Invalid Service Parameter" from Amazon Product Advertising API

I'm working on a website that is highly tied to Amazon's Product Advertising API. We're using the CartCreate operation to send our users over to Amazon to add books that they've found on our site over to Amazon.com to complete their purchase.

Over the last two hours or so, the CartCreate operation has ceased to function for us. Here's an example request.

GET ecs.amazonaws.com/onca/xml?
AWSAccessKeyId=XXXXX
&AssociateTag=XXXXX
&Item.1.OfferListingId=ZtjdY2eWnf3lmAPcG%252FvpjeLlkWjkvzIx%252F83ChfeC%252Bb%252FKBTIH2o6A%252FAiDT6LFHA%252BgIge9llxK7AnvmFtz6qlpXqFBC2%252FHN09XTMbhXaBRdEDrxfL0znQIHj8BlJN%252B1%252F%252BQ634mTrEZCao%253D
&Item.1.Quantity=1
&MerchantId=All
&Operation=CartCreate
&Service=AWSECommerceService
&Timestamp=2011-01-11T23%3A58%3A05Z
&Version=2010-11-01
&Signature=DCNKITwBIvJHJiPFTFWTjJTPY763iM5v9J2FgJmnVaU%3D

This worked perfectly up until about 3:00pm PST. I would get an XML response with a Cart.PurchaseURL parameter to direct the user's to complete the checkout.

Now, I'm getting this:

<Errors>
  <Error>
    <Code>AWS.InvalidServiceParameter</Code>
    <Message>The Service  parameter is invalid. Please modify the Service parameter and retry.</Message>
  </Error>
</Error开发者_运维百科s>

You can see that the Service parameter is included in my request. Has Amazon changed the API over the last hour, or is there some new documentation that I'm not taking into account? Help is much appreciated, thanks!

Dan