Yeah, "Prova Wind" is a promotion for the mobile number portability. Of course you can buy a Vodafone IT prepaid sim-card (they are on offer at 5 euros) and ask Wind the MNP with this promotion and you get double recharges. Once you move to Wind, keeping your Vodafone number, any time you recharge you get the recharged airtime at once, and then the following month you get the same recharged airtime once again, for free (max 50 euros per month; consider that 25 eur recharge has 20 eur of airtime). I have to inform myself, but I think there are no limitations in the free credit, so it should be "normal credit" which you can use for the Mega No-Limit option as well.
If you want to do that choose Vodafone since they are much better than TIM in "loosing" clients!
And it's also cheaper by now.
Answering to DRNewcomb: if you want to use wap you have to use wap.wind, which is the official gateway. You can use it with the Mega NoLimit option, no problems, except that you pay 10 times more than with the web gateway. I don't know any 3rd party gateway which can be used paying the web rate. You can try the i-mode gateway as well, it's 3 times cheaper than wap and it could work for you.