Wireless carriers have long been known to suspend or cancel unlimited wireless data accounts for what they deem exorbitant transfer, or place relatively high "fair usage" caps on "unlimited" data plans. O2, the UK carrier for the iPhone, however, is reportedly taking the cap on "unlimited" data to a new low.
The company has, according to a ZDNet report, placed a 200MB "fair use" cap on unlimited(*) data plans sold in conjunction with the iPhone.
While this limit doesn't mean that your O2 iPhone's service will necessarily be cut off after 200MB of transfer through GPRS, nor does it indicate that there will be an additional charge if the amount is exceeded, it does give the carrier the right to suspend or cancel your account if data usage consistently exceeds this amount.
200MB may be an adequate limit for light Web browsing and email retrieval, but it might make some O2 customers pause before downloading videos through the iPhone's YouTube application or viewing large PDF attachments on the road.