I can negate the need for the tinfoil hat with a small amount of authority, my current company builds smart meters, mostly for the US market and the software
to run many of them (not naming names but if you really want to know look up cell based meters, should find us) and the simple fact is that with that type
of meter the power profile lacks the resolution for such snooping, the most often I have seen the usage profile read is once an hour, and that (probably) is
not enough info to do what they are doing. (H-online is down for me as I write this, I will try again later to read it) In the type of meter's I see such detailed
information is just that, too detailed. A 2mb a month data plan is the typical provisioning and the meters generally use half that in a given month.
Note wireless mesh meters may be another story, at least in some deployments, however the only such network like that I have any direct experience with
uses a cell based relay station to phone home with and again doesn't seem to send enough data for such evil observations to be made.
End result? You could do it, but real life smart meter systems have one giant consideration that stops this from working, cost effectiveness, power companies
don't want to pay for data, that just want to know how much of your money to take.