A semi little-known fact... as of the 1st January 2007, the rules for UK companies regarding business stationery changed. Just like every registered company is bound to include certain information (the registered office, the geography of registration (eg England & Wales) and its company registration number) on all its official letters & order forms, electronic communications now fall under this rule.
As Companies House says:
Whenever an email is used where its paper equivalent would be caught by the stationery requirements then that email is also subject to the requirements.
I can honestly only think of one case where a company includes all this stuff in their email, along with a long-winded disclaimer. I suppose the rules are now in place and people are waiting to see how they're interpreted... might be worth thinking about including your details on your own e-mail .sig...
There's quite a good discussion of the whole area on legal eagles Pinsent Masons site, here.
Oh, and did you know that Exchange 2007 now has the ability to include standard disclaimers on all mail that passes through it? For a step-by-step illustration, have a look over on msexchange.org.