I'd probably not attribute HP, but I might attribute Mathematica if I used results of computations that are non-trivial (not easily recreatable without Mathematica).
I believe it will sort itself out with Wolfram Alpha. Normally you'd try to go to the primary sources and attribute them (just like you'd do with Wikipedia and Google). And simple calculations could be done by any means, so attribution is not necessary (the presented results cannot be traced back to Wolfram Alpha).
But for some of the more specialized queries where you let Wolfram Alpha combine information from multiple sources and perform computations on them, you will want to add an attribution. The sources section of Wolfram Alpha is sufficiently vague and the computations sometimes non-transparent, that you are basically trusting Wolfram Alpha on this stuff, and your readers deserve to know this.
It also serves as an insurance if the information happens to be wrong. Wrong information presented with attribution is the fault of the source, wrong information given without attribution is your responsibility.