I've been trying to upload around 210 million items to Amazon's SimpleDB service, which has been quite an adventure! Sid Anand's advice has been invaluable (he's done an even larger migration of data for Netflix), and I'll be blogging in more depth on the details, but one of the early problems I hit was the lack of any easy way to interact with the store. With MySQL you at least get a console you can use to sanity check your results, but SimpleDB was a black box.
Eventually I discovered a handy solution, SimpleDB Explorer. It's a commercial product, but comes with a free 30 day trial and only costs $35. I loathe Java for GUIs, and it does have some quirks like over-enthusiastic dialogs that pop up willy-nilly, but it does run on Windows, Linux and OS X. It's got the functionality you'd expect, you can edit the overall structure of the store, run queries or just browse the data to make sure it looks reasonable. It's saved me a lot of time, if you're doing any serious work with SimpleDB I'd highly recommend buying it.