I'm wondering about that too. It seems Nuance (the platform Siri is built on) supports lots of accents, though I'm not sure how this works on the iPhone. Maybe it uses your regional settings to choose an accent profile?
The voice seems to depend on the language. You seemingly can't explicitly switch it, you only can switch languages. It just so happens that iOS has both 'English' and 'British English' as languages, with female and male voice synthesis respectivly. (Those languages are already built into the current version of iOS as part of VoiceOver.)
If you are German or French you only ever get one voice.
http://community.nuance.com/blogs/dragon/archive/2009/03/05/...