I believe the iphone web and wifi bolt on is truely unlimited, but the o2 web bolt on has an FUP of 200mb.
hope this helps.
T's and C's web and wifi.
The O2 Web & Wi-Fi Bolt On allows you unlimited use of O2 UK's Edge, GPRS, 3G and HSDPA networks, The Cloud's UK Wireless LAN network and the BT Openzone UK Wireless LAN network, for personal internet use and email. All usage must be for your private, personal and non-commercial purposes. You may not use your SIM Card in any device to allow the continuous streaming of any audio / video content, enable P2P or file sharing or use them in such a way that adversely impacts the service to other O2 customers. If O2 reasonably suspect you are not acting in accordance with this policy O2 reserves the right to impose further charges or disconnect your tariff at any time, having attempted to contact you first.
T's and C's o2 web bolt on
The O2 Web Bolt On can only be used for web browsing on your phone and a fair use policy applies. But if your usage exceeds that of other users, which is normally expected to be below 200MB a month, you'll be charged standard rates for additional browsing. We'll send you a warning text message advising you to limit your usage before this happens, and a text message to let you know when you're being charged.