For those of you that don't know, recently Google introduced a whole bunch of improvements to its Gmail service, including "chat" integration (providing part of the functionality of Google Talk) and searchable chat logs right inside your Gmail window, with all sorts of AJAXy crap in there.
However, all of my contacts seemed to have got these upgrades, while I've been stuck with the older version of Gmail. I could select to have my chats saved in Gmail from Google Talk, but I couldn't actually access them!
I finally found out what the problem is. For some reason, the Chat functionality (and Web Clips, and Quick Contacts) are only activated if you have the language set to US English. So, if you had similar problems, I strongly suggest setting the language back to US (I had mine set to UK).
I'll be leaving mine on US to make sure I get any future upgrades straight away!