One of the confusing areas for anyone is what 'unlimited' data tarrifs really mean.  I'm in the industry and it's confusing for me let alone any consumer or corporate who wants to minimise their risk and exposure to massive data bills through the use of unlimited tarrifs. 

Now every operator protects the use of the term 'unlimited' by assigning a fair use policy.  What is interesting is that different operators have different fair use policies.  This is highlighted in an article on ZDNet where they compare the iPhone O2 tarrif and the T-Mobile Web 'n' Walk Tarrif.

O2's fair use policy allows the download of 1400 Internet pages where they claim an Internet page is 4-5kb.  In fact the O2 Homepage is 33kb.

So for 1400x5kb you actually get 200MB per month of data.

T-Mobile on the other hand state fair use to be 1GB per month.

If Operators are really serious about driving the uptake of mobile data they really need to make it clear that unlimited really means 200MB or 1GB respectively.  Similarly operators need to report to customers how much they are using proactively so you can monitor it.

My car has an unlimited use capability where the fair use is set to 400 miles. 

The ZDNet article goes into a lot more detail around this HERE