Are you using your original Nokia N900 wall charger (AC-10U)? There is a known peculiarity with N900 in that it won't charge from the majority of existing generic 3rd party USB chargers, car kind including. Workarounds do exist however, search for "charging from dumb power sources" on this forum.

In my case the AC-10U was the culprit, got a direct replacement from Amazon for $11 and everything is back to normal. At the same time I'm trying to get a warranty replacement from Nokia, and the process is not smooth, but that's a whole different story.

BTW, to answer my original question, AC-10U does charge Kindle.