So I'm now hosting on a US server which is, of course, configured for US date formats, number formats etc. Whilst subtext has the option to set a culture it, err, doesn't work. Of course being open source (BSD licensed, so you can do what you want with it) the code is available, but there is a lazy way whilst you wait for 2.0 to arrive.
Open up your web.config file and look for the system.web section. Near the top you should see
<globalization
culture="en-US"
requestEncoding="utf-8"
responseEncoding="utf-8" />
Simply change the culture to the one you desire (en-GB in my case) and the dates and numbers will format correctly. If there isn't a globalization entry then just add one of your own.