Just did a bit of research into what this is and just wanted to share with you guys.
If you take a Simplicity tariff from O2, you can use 100MB data for £1 / day. i.e. you can use 30 x 100 = 3000 , 3GB / month!
Of course, you do end up paying £30pm
The cheapest sensible Simplicity plan is £15pm > 600 mins. So, you end up paying £45pm but get 3GB data.
This might be ideal for some iPhone users or those of you who are getting 500MB or 1GB cap ?