If you have fallen in love with your collections on your Kindle, you will appreciate finally having them in your Kindle for your PC or Mac. When you install the latest update, you will see a choice to import your collections from your Kindle (or Kindles if you have more than one). Now, the actual books won't be imported, but placeholder/shortcuts to the titles in each collection you have will be listed. Clicking them will cause each to be downloaded as you want them.
I LOVE it! Haven't had much of a chance to tinker with it yet, but I can see it as a pretty decent way to "backdoor backup" your collections. Any time you can put them in more than one place, you are less likely to lose them in case of (gasp!) a disaster.
Not such a happy story for the Kindle app for your iPad or iPhone... And the Kindle Cloud is still in its baby stages... no collections available for them yet. Maybe soon?