I know it's been posted before and expired, but they're running it again, but only till Thursday 23/06/2011 at 11:59pm!
Deal link can't be updated, so either paste this in http://direct.tesco.com/q/R.211-2318.aspx
to your browser, or use Rock2007's link in the comments. Use code TDX-KYRA at checkout to get £20 off.
This only applies to the 3G model, as the voucher only works for spends above £150.
**UPDATE, apparantly Tesco will not accept the code on the phone, it does work online, thanks to MPW33CFR for the feedback on that one***
The offer's only available until 11.59pm Thursday 23 June and you'll need to collect it at your nearest Tesco to avoid delivery cost.
Tesco has said there are currently 800 3G Kindles in stock; if it runs out 1,500 more will be available from Thursday.
Saw via MSE.