Well, a bit more experimenting has confirmed my fear that the out-of-the-box configuration of the iPhone 3G has disappointing battery life - use it heavily (as you undoubtedly will) and it won't even see you through a day.
Fortunately, there's a simple solution that I was alerted to by a colleague (cheers Mark). There are two steps:
1) Reset your iPhone's settings. Do this by going to 'Settings/General/Reset'. This won't delete anything too important, although it did remove my Exchange email settings (though fortunately not the security certificate I need to access the Microsoft Exchange server).
2) Turn off 'Location Services'. This essentially de-activates the GPS, thereby doubling your iPhone's battery life.
There are other things you can do to improve battery life - make sure 'Auto-Lock' is set to '1 minute in 'Settings/General', and reduce the brightness of your screen in 'Settings/Brightness'. The iPhone 3G's screen is noticeably brighter than the old iPhone's so you're not losing anything by doing this - and you can always boost the brightness if and when you need to.
UPDATE Apple also has an official battery optimisation page here.