First off, I freely admit that I'm kind of bad at picking up social cues sometimes.
But it seems to me that since your wife has mentioned that she'd like to have a Kindle specifically, with the particular colour she wants also specified, within recent weeks, even if there's no anniversary mention to go with it, it might be A Hint.
If nothing else, you now know that it's something she actually wants.
If you think perhaps this is not romantic enough for an anniversary or that she might think it's not romantic enough and wants jewelry/a spa vacation or whatever, then maybe you could fancy it up with a candlelit dinner (home-cooked or restaurant, depending on your budget) in which you present the nicely-wrapped Kindle.
If she makes another mention of wanting the Kindle in future conversations, then I'd say it's safe to say that it's definitely A Hint and you should get her one.
If not, you could always try and ask her friends if she's mentioned anything to them about any expectations/hopes she might have regarding the anniversary and adjust your plans accordingly.