TBL has already apologised for that:
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It depends on your CPU scheduler and your throttling algorithm, too.
I run BOINC on linux. BOINC is "niced" to have an idle priority, meaning that CPU time is only granted to it if there's nothing better to be doing. In addition, I used the on-demand frequency governor which I have instructed to ignore "niced" processes when determining whether to spin up the CPU.
As a result, yes, BOINC only uses spare CPU cycles and not too many of them, either.
What's the point of a Tablet? Well the market for PDAs has basically disappeared. If you don't fancy paying ~£800 for your handheld computer (£30/month contract for 2 years plus the initial cost), Tablets are the way to go.
The linked ICANN blog suggests http://xn----rmckbbajlc6dj7bxne2c.xn--wgbh1c/ as an example.