3G mobile data cards and even higher speed services using HSPDA are gaining momentum, but it would appear O2 has yet to embrace the full potential of giving people access to the full speed of 3G let alone the faster speeds other companies can provide.
The Register has the detail on this limit to 128Kbps, which apparently applies to all 3G connections even if you are getting a signal that would support a 384Kbps connection. Some business users are upgraded to 384Kbps, though how you request this or what the criteria are has not been explained by O2.
For someone just checking email on a mobile phone, 128Kbps is possibly plenty, but one of the big advantages of internet access when mobile is access to online maps which can involve fairly large images. One apparent irony of this 3G limit appears to be that the Edge connection which iPhone uses may out perform an O2 3G connection.
Some websites detect the browser being used to access the site and adjust content appropriately but with the use of 3G data cards on laptops you would get served the full fat version. When the main frontpage of a fairly simple site like Channel 4 News weighs in at 200KB, this would take around 12 seconds to download on an O2 3G service, dropping to potentially 4 seconds if the signal is good enough on other networks.