A Wi-Fi only version of the iPad 2 costs £100 less than the iPad 2 3G version and should normally have a better battery life compared to the latter; hence the interest in Mi-Fi.
The only major mobile phone operator that is actively backing Mi-Fi is Three Mobile and it would not be surprising if they sell the iPad 2 Wi-Fi with a Mi-Fi dongle.
After all, a subsidised 16GB Wi-Fi version would almost certainly be £100 cheaper than the current offer (£25 per month on a two year contract with £229 initial cost) plus a Mi-Fi device is more flexible than an iPad 2 only mobile data allowance.
For a start, a Mi-Fi device is a mobile wireless hotspot which can serve more than just one device which makes it ideal if you cannot use up all of the 15GB data allowance Three Mobile allocates to its cheapest bundle (in fact all of them).
Then, Three Mobile's Mi-Fi device can also double up as an external mobile storage peripheral which comes handy when having to read a microSD card for example.
As for the price, Three Mobile charges £16 for 15GB mobile broadband on a two year contract (dongle version), £18 for the same model but with the Mi-Fi and £25 for the iPad 2 version.
This means that the a Wi-Fi only iPad 2 with Mi-Fi would cost around £2 more per month compared to the current Three Mobile deal but with a cheaper initial outlay. In other words, £27 per month on a two year contract with a £129 initial cost assuming the same level of subsidies.