I've just got a purpose made on-board camera from
http://www.4Kam.com. Its really good for mounting externally in interesting places like behind the wheel arch etc.
It's a good quality camera with 520 tvl and comes with a bunch of stuff including a separate mic which is handy and it costs less than £100.
Anyway if you ever wanted an on board cam then I recommend it. Good service and cracking value for money.
J