Hi,
'Everywhere you go' - I'll take that as meaning having internet access on your laptop as you move around, visit places, sit down in public places/buildings etc etc.
You need a HSDPA equipped PCMCIA data card which is active on a suitable airtime plan from one of the mobile phone service providers.
HSDPA currently on Vodafone's tarriffs gives you about 1.3M/Sec internet connectivity whilst out and about, I think T mobiles 'webnwalk' plan offers similar. The mobile phone companies are always developing, so expect to see those speeds increase quite substantially over the near future as they bump up performance etc.
I've actually never tried this out myself, I don't need constant connectivity at the moment, but have been looking into it as this may change for me.
I suggest you go into a mobile phone store from each of the networks, and find out about thier plans and the hardware needed for your particular requirements.
There are 'alternatives' - A suitable PDA/Phone for instance can have HSDPA connectivity - not many models around that do, but are there (Xda TyTN - being the one I can think of - it is sold under different names depending on provider) so If all you want is access to get emails something like that would suit well. If you want 'internet access' to surf, you may find looking at a small screen like that would be fidgety after a while and so you'd need a ultra portable laptop or sub notebook or 'normal sized' laptop. Depends on your requirements and budget really.
Overall, it's not hard nowadays to have roaming, broadband speed internet connectivity - just watch out for the charges for the amount of data you'll be transferring, if you go over your bundled allowance, it can add on substantial chunks to your monthly airtime bill.
Good luck! Cheers, DtM! West London & Slough UK!