The One Plan SIM cards DO allow tethering (ALL 3 contract's do, i think)
And they will work in a dongle/MiFi/iPad.... for about 5 minutes.
then you get a lovely message telling you that it's not supported. I know, i tried.
A bit of a pain, but then again, 3's data only cards are far far cheaper than everyone else's so it's not all bad.
As for me, One plan SIM in the iPhone - WiFi tether to the iPad - no second card to pay for. Happy days!!
Getting about 15mb down and 900k up on Sky DSL, not bad when it's free! Can't wait for FTTC (30-6-12)
Disclaimer: I work for sky, but have no involvement AT ALL with broadband, so dont ask!