Im very intrigued with this. Iv never been too happy with the Nokia N900 due to the FUP 500MB, and this new introduction makes life worse. So much so, that I just use my iPhone to browse due to lower data usage.

This is what Tom_Vodafone had to say (Vodafone eForum post #297):
Will I be allowed to cancel my account under Clause 7?
We’ ll be giving you 14 days' notice before introducing Out Of Bundle charging which complies with clause 7a. You'll be entitled to end your contract if you can show that the introduction of the new charges has increased your total call and usage charges by more than 10%. This needs to be compared to the same amount of usage in the previous month. You must also write to us within the 14 day window indicating that you want to end your contract. (Address: Vodafone UK, Vodafone House, The Connection, Newbury, Berkshire RG14 2FN).

I've asked and just looking to confirm if the comparison of 10% on previous month is May bill (which is still in progess) against the April bill.
If YES, then thats plenty of time to up the data usage to incur a small charge.
Or, if it isnt May vs April, then it would be June vs May...and of course come 1st June you start getting charged....no doubt it will be more than 10%.